Remove definitions duplicated from drm_mode.h
I don't exactly know why these were duplicated before. Maybe libdrm didn't always vendored drm_mode.h from the kernel? In any case, we now do, so instead of having copy-pasted definitions, just include our vendored version which cannot be outdated.
Contrary to what the comment says, drm.h doesn't include drm_mode.h, so we need to add the include.
Signed-off-by: Simon Ser contact@emersion.fr