Adds docs for pixel_blend_mode in drm_plane_state. Fixes the warning
found by kbuild test robot:
htmldocs: include/drm/drm_plane.h:189: warning: Function parameter or member 'pixel_blend_mode' not described in 'drm_plane_state'
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Lowry Li <lowry.li@arm.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180831150934.202332-1-sean@poorly.run
         * details.
         */
        u16 alpha;
+
+       /**
+        * @pixel_blend_mode:
+        * The alpha blending equation selection, describing how the pixels from
+        * the current plane are composited with the background. Value can be
+        * one of DRM_MODE_BLEND_*
+        */
        uint16_t pixel_blend_mode;
 
        /**