drm: add fourcc codes for 16bit R and RG

This adds fourcc codes for 16bit planes required for DRM buffer
export to mesa.

Signed-off-by: Rainer Hochecker <fernetmenta@online.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20170104183855.3852-1-fernetmenta@kodi.tv


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>