dri: add loader_dri_create_image helper

dri: add loader_dri_create_image helper

The DRI image extension already has two different ways to allocate an
image (with and without a modifier) and will soon grow a third one.
Add a helper, which handles calling the appropriate implementation to
get rid of code duplication in the winsys.

This convert the two obvious call sites (GBM dri and EGL wayland)
that profit from the code dedup.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8106>