dri: Deduplicate the image fourcc macros
Continuing my format enum unification quest, I needed to understand the endian behavior of DRI images, and it would have been easier if I had one less level of indirection to drm_fourcc.h.
(Also, this should help keep people honest with not defining local fourccs that might later clash wit the kernel)
This needs a pigliting, or maybe Intel CTS.