xwayland: only use linux-dmabuf if format/modifier was advertised

Previously, linux-dmabuf was used unconditionally if the buffer had a
modifier. However creating a linux-dmabuf buffer with a format/modifier
which hasn't been advertised will fail.

Change xwl_glamor_gbm_get_wl_buffer_for_pixmap to use linux-dmabuf when
the format/modifier has been advertised only.

Signed-off-by: Simon Ser <contact@emersion.fr>
Closes: #1035


Tested-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>