meson: DRM-backend demands GBM

All the GBM code is unconditional in compositor-drm.c, so while disabling the
GL-renderer would stop GBM from being used, GBM headers would still be needed
for building and GBM library for linking.

Leave a note to fix it properly later. At least we now check for GBM and do not
mislead with the error message.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
(cherry picked from commit fe6dd7bc)