drivers: gpu: Mark functions as static and remove unused function in cirrus_ttm.c

Mark functions cirrus_ttm_global_release(), cirrus_ttm_bo_is_cirrus_bo()
and cirrus_ttm_tt_create() as static in drm/cirrus/cirrus_ttm.c because
they are not used outside this file. Remove unused function
cirrus_bo_unpin() from drm/cirrus/cirrus_ttm.c.

This eliminates the following warnings in drm/cirrus/cirrus_ttm.c:
drivers/gpu/drm/cirrus/cirrus_ttm.c:84:1: warning: no previous prototype for ‘cirrus_ttm_global_release’ [-Wmissing-prototypes]
drivers/gpu/drm/cirrus/cirrus_ttm.c:105:6: warning: no previous prototype for ‘cirrus_ttm_bo_is_cirrus_bo’ [-Wmissing-prototypes]
drivers/gpu/drm/cirrus/cirrus_ttm.c:211:16: warning: no previous prototype for ‘cirrus_ttm_tt_create’ [-Wmissing-prototypes]
drivers/gpu/drm/cirrus/cirrus_ttm.c:378:5: warning: no previous prototype for ‘cirrus_bo_unpin’ [-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>