drm/qxl: fix prime vmap

Use the correct vmap variant.  We don't have a reservation here,
so we can't use the _locked version.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210217123213.2199186-8-kraxel@redhat.com