drm: Add missing documentation for struct drm_plane_size_hint

This patch takes care of the following warnings during documentation
compiling:

./include/uapi/drm/drm_mode.h:869: warning: Function parameter or struct member 'width' not described in 'drm_plane_size_hint'
./include/uapi/drm/drm_mode.h:869: warning: Function parameter or struct member 'height' not described in 'drm_plane_size_hint'
Signed-off-by: Mohammed Anees <pvmohammedanees2003@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240811101653.170223-1-pvmohammedanees2003@gmail.com

include/uapi/drm/drm_mode.h

@@ -859,6 +859,8 @@ struct drm_color_lut {
 
 /**
  * struct drm_plane_size_hint - Plane size hints
+ * @width: The width of the plane in pixel
+ * @height: The height of the plane in pixel
  *
  * The plane SIZE_HINTS property blob contains an
  * array of struct drm_plane_size_hint.