drm/fourcc: Add formats R8, RG88, GR88

The Kodi/XBMC developers want to transcode NV12 to RGB with OpenGL
shaders, importing the two source planes through
EGL_EXT_image_dma_buf_import. That requires importing the Y plane as an
R8 EGLImage and the UV plane as either an RG88 or GR88 EGLImage.

CC: Peter Frühberger <peter.fruehberger@gmail.com>
Cc: Rainer Hochecker <rainer.hochecker@onlinehome.de>
Cc: Benjamin Widawsky <benjamin.widawsky@intel.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Chad Versace <chad.versace@intel.com>