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vendored version which cannot be outdated.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>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>