drivers: gpu: Mark function as static in sis_drv.c

Mark function as static because it is not used outside the file
drm/sis/sis_drv.c.

This eliminates the following warning in drm/sis/sis_drv.c:
drivers/gpu/drm/sis/sis_drv.c:97:6: warning: no previous prototype for ‘sis_driver_postclose’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>