drm/rockchip: vop: Quiet always-warning AFBC log

The downstream code from which this was derived didn't ever run through
this 'switch' block with non-AFBC formats, but the upstream code does --
we use this function to probe whether a given format is supported.

Demote the warning to eliminate this sort of warning seen on every
boot:

  [drm] unsupported AFBC format[3231564e]

And make it warn more than once, because if we *actually* care to see
what formats we're probing/rejecting and for what reasons, we probably
care about more than just the first message.

Drop the comment, because one of the two *is* commonly reachable.

And lastly, drop the unreachable return; we'd do better to let the
compiler complain if we start hitting this unexpectedly.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221031101557.1.Ic1569d394173c1c3016142fee4bb87a09753db94@changeid

drivers/gpu/drm/rockchip/rockchip_drm_vop.c

@@ -316,13 +316,10 @@ static int vop_convert_afbc_format(uint32_t format)
 	case DRM_FORMAT_RGB565:
 	case DRM_FORMAT_BGR565:
 		return AFBC_FMT_RGB565;
-	/* either of the below should not be reachable */
 	default:
-		DRM_WARN_ONCE("unsupported AFBC format[%08x]\n", format);
+		DRM_DEBUG_KMS("unsupported AFBC format[%08x]\n", format);
 		return -EINVAL;
 	}
-
-	return -EINVAL;
 }
 
 static uint16_t scl_vop_cal_scale(enum scale_mode mode, uint32_t src,