drm: Add missing documentation for struct drm_plane_size_hint

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 <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]