drm: remove unused header gma_drm.h

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

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240408104230.3191827-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>