gbm/dri: Propagate queryDmaBufModifiers return value

gbm/dri: Propagate queryDmaBufModifiers return value

We were treating count == 0 as the format not being supported at all,
but queryDmaBufModifiers would return false in that case.

Fixes spuriously reporting all formats as unsupported with radeonsi
(which doesn't support modifiers yet), which would e.g. cause mutter
to think the HW cursor format isn't supported and fall back to SW
cursor.

Suggested-by: Daniel Stone <daniels@collabora.com>
Fixes: 4e3a7dcf6ee4 "gallium: enable
                     EGL_EXT_image_dma_buf_import_modifiers
                     unconditionally"

Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4532>