gbm: implement FD import with modifier

gbm: implement FD import with modifier

This implements a way to import FDs with modifiers on plain GBM devices,
without the need to go through EGL. This is mostly to the benefit of
gbm_gralloc, which can keep its dependencies low.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>