Split up dri_interface.h

(Includes !16377 (merged) which hasn't marged yet, also I haven't actually tried to rebuild xserver with this yet but grep says I should succeed.)

dri_interface.h is a Mesa-specific quirk that only xserver needs, at this point, and better solutions are available in EGL. This strips dri_interface.h to the bare minimum needed to build xserver, making it nice and clear that the rest is internal ABI and subject to change.