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]