Fixes#11257 by ensuring winsys mapping functions is only called
if its supported by the winsys, which should prevent llvmpipe from
crashing with kmswast.
If the winsys is kms_swrast then this method will be null, but on
drisw it will be available.
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29546>
db38a4913e