drm: Describe pixel_blend_mode in drm_plane_state

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 <[email protected]>
Cc: Lowry Li <[email protected]>
Reviewed-by: Maarten Lankhorst <[email protected]>
Signed-off-by: Sean Paul <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: atseanpaul

include/drm/drm_plane.h

@@ -117,6 +117,13 @@ struct drm_plane_state {
 	 * 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;
 
 	/**