drm/qxl: add qxl_bo_vmap/qxl_bo_vunmap

Add vmap/vunmap variants which reserve (and pin) the bo.
They can be used in case the caller doesn't hold a reservation
for the bo.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210217123213.2199186-7-kraxel@redhat.com