drm/i915: Widen return value for reservation_object_wait_timeout_rcu to long.

This fixes a spurious warning from an integer overflow on 64-bits systems.
The function may return MAX_SCHEDULE_TIMEOUT which gets truncated to -1.

Explicitly handling this by casting to lret fixes it.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reported-and-tested-by: Joseph Yasi <joe.yasi@gmail.com>
Tested-by: Andreas Reis <andreas.reis@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: drm-intel-fixes@lists.freedesktop.org
Fixes: 3c28ff2 ("i915: wait for fence in prepare_plane_fb")
Link: http://patchwork.freedesktop.org/patch/msgid/5666EEC8.2000403@linux.intel.com

Author: mlankhorst

drivers/gpu/drm/i915/intel_display.c

@@ -13902,13 +13902,15 @@ intel_prepare_plane_fb(struct drm_plane *plane,
 
 	/* For framebuffer backed by dmabuf, wait for fence */
 	if (obj && obj->base.dma_buf) {
-		ret = reservation_object_wait_timeout_rcu(obj->base.dma_buf->resv,
-							  false, true,
-							  MAX_SCHEDULE_TIMEOUT);
-		if (ret == -ERESTARTSYS)
-			return ret;
+		long lret;
+
+		lret = reservation_object_wait_timeout_rcu(obj->base.dma_buf->resv,
+							   false, true,
+							   MAX_SCHEDULE_TIMEOUT);
+		if (lret == -ERESTARTSYS)
+			return lret;
 
-		WARN_ON(ret < 0);
+		WARN(lret < 0, "waiting returns %li\n", lret);
 	}
 
 	if (!obj) {