Mark function qxl_device_init() as static in drm/qxl/qxl_kms.c because
it is not used outside this file.

This eliminates the following warning in drm/qxl/qxl_kms.c:
drivers/gpu/drm/qxl/qxl_kms.c:118:5: warning: no previous prototype for ‘qxl_device_init’ [-Wmissing-prototypes]
Signed-off-by: NRashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NDave Airlie <airlied@redhat.com>