drm: document minimum kernel version for DRM_CLIENT_CAP_*

The kernel versions including the following commits are referenced:

DRM_CLIENT_CAP_STEREO_3D
61d8e32 ("drm: Add a STEREO_3D capability to the SET_CLIENT_CAP ioctl")

DRM_CLIENT_CAP_UNIVERSAL_PLANES
681e7ec ("drm: Allow userspace to ask for universal plane list (v2)")
c7dbc6c ("drm: Remove command line guard for universal planes")

DRM_CLIENT_CAP_ATOMIC
88a48e2 ("drm: add atomic properties")
8b72ce1 ("drm: Always enable atomic API")

DRM_CLIENT_CAP_ASPECT_RATIO
7595bda ("drm: Add DRM client cap for aspect-ratio")

DRM_CLIENT_CAP_WRITEBACK_CONNECTORS
d67b6a2 ("drm: writeback: Add client capability for exposing writeback connectors")

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Daniel Stone <daniels@collabora.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/434202/

include/uapi/drm/drm.h

@@ -780,6 +780,9 @@ struct drm_get_cap {
  * If set to 1, the DRM core will expose the stereo 3D capabilities of the
  * monitor by advertising the supported 3D layouts in the flags of struct
  * drm_mode_modeinfo. See ``DRM_MODE_FLAG_3D_*``.
+ *
+ * This capability is always supported for all drivers starting from kernel
+ * version 3.13.
  */
 #define DRM_CLIENT_CAP_STEREO_3D	1
 
@@ -788,6 +791,9 @@ struct drm_get_cap {
  *
  * If set to 1, the DRM core will expose all planes (overlay, primary, and
  * cursor) to userspace.
+ *
+ * This capability has been introduced in kernel version 3.15. Starting from
+ * kernel version 3.17, this capability is always supported for all drivers.
  */
 #define DRM_CLIENT_CAP_UNIVERSAL_PLANES  2
 
@@ -797,6 +803,13 @@ struct drm_get_cap {
  * If set to 1, the DRM core will expose atomic properties to userspace. This
  * implicitly enables &DRM_CLIENT_CAP_UNIVERSAL_PLANES and
  * &DRM_CLIENT_CAP_ASPECT_RATIO.
+ *
+ * If the driver doesn't support atomic mode-setting, enabling this capability
+ * will fail with -EOPNOTSUPP.
+ *
+ * This capability has been introduced in kernel version 4.0. Starting from
+ * kernel version 4.2, this capability is always supported for atomic-capable
+ * drivers.
  */
 #define DRM_CLIENT_CAP_ATOMIC	3
 
@@ -805,6 +818,9 @@ struct drm_get_cap {
  *
  * If set to 1, the DRM core will provide aspect ratio information in modes.
  * See ``DRM_MODE_FLAG_PIC_AR_*``.
+ *
+ * This capability is always supported for all drivers starting from kernel
+ * version 4.18.
  */
 #define DRM_CLIENT_CAP_ASPECT_RATIO    4
 
@@ -814,6 +830,9 @@ struct drm_get_cap {
  * If set to 1, the DRM core will expose special connectors to be used for
  * writing back to memory the scene setup in the commit. The client must enable
  * &DRM_CLIENT_CAP_ATOMIC first.
+ *
+ * This capability is always supported for atomic-capable drivers starting from
+ * kernel version 4.19.
  */
 #define DRM_CLIENT_CAP_WRITEBACK_CONNECTORS	5