gbm: add gbm_bo_get_fd_for_plane

gbm: add gbm_bo_get_fd_for_plane

This commit adds a new gbm_bo_get_fd_for_plane function, which does the
same as gbm_bo_get_fd but allows specifying the plane.

v2: - Rename to gbm_bo_get_fd_for_plane (Emil)

Signed-off-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5442>