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 <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