ui: Add dpy_gfx_check_format() to check backend shared surface support

This allows VGA to decide whether to use a shared surface based on
whether the UI backend supports the format or not. Backends that
don't provide the new callback fallback to native 32 bpp which
is equivalent to what was supported before.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

[ kraxel: fix console check, allow only 32 bpp as fallback ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>