drm/qxl: fix monitors object vmap

Use the correct vmap variant.  We don't hold a reservation here,
so we can't use the _locked variant.  We can drop the pin because
qxl_bo_vmap will do that for us.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210217123213.2199186-9-kraxel@redhat.com