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

Mark function cirrus_bo_unref() as static in drm/cirrus/cirrus_main.c
because it is not used outside this file.

This eliminates the following warning in drm/cirrus/cirrus_main.c:
drivers/gpu/drm/cirrus/cirrus_main.c:258:6: warning: no previous
prototype for ‘cirrus_bo_unref’ [-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>