st/dri: enable DRIimage modifier queries

st/dri: enable DRIimage modifier queries

return the modifier selected by the driver when creating this image.

v2: since we can use winsys_handle->modifier to serve these, remove
    DRIimage->modifier from v1.
    use DRM_API_HANDLE_TYPE_KMS instead of DRM_API_HANDLE_TYPE_FD to avoid
    ownership transfer. (Lucas)

Suggested-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Varad Gautam <varad.gautam@collabora.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>