qxl: No need for NULL check before calling qxl_bo_unref()

qxl_bo_unref() is already performing a NULL check.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181120162004.22807-1-cfergeau@redhat.com


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>