drm: remove unused header gma_drm.h

gma_drm.h has become an empty, unused header. Remove.

Cc: Patrik Jakobsson <[email protected]>
Reviewed-by: Javier Martinez Canillas <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Jani Nikula <[email protected]>

Author: jnikula