virgl: adjust strides when mapping temp-resources

When we're mapping temp-resources, we clip the resource to the
transfer-box, which means the stride might not be correct any more.

So let's update the stride from the temp-resource, and recompute the
layer-stride.

This fixes crashes when running dEQP with --deqp-gl-config-name=rgba8888d24s8ms4

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Fixes: a8987b88 "virgl: add driver for virtio-gpu 3D (v2)"
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit fa5e9f1f)