drivers: gpu: Mark functions as static in mgag200_ttm.c

Mark functions mgag200_ttm_global_release(),
mgag200_ttm_bo_is_mgag200_bo() and mgag200_ttm_tt_create() as static in
drm/mgag200/mgag200_ttm.c because they are not used outside this file.

This eliminates the following warning in drm/mgag200/mgag200_ttm.c:
drivers/gpu/drm/mgag200/mgag200_ttm.c:84:1: warning: no previous prototype for ‘mgag200_ttm_global_release’ [-Wmissing-prototypes]
drivers/gpu/drm/mgag200/mgag200_ttm.c:105:6: warning: no previous prototype for ‘mgag200_ttm_bo_is_mgag200_bo’ [-Wmissing-prototypes]
drivers/gpu/drm/mgag200/mgag200_ttm.c:211:16: warning: no previous prototype for ‘mgag200_ttm_tt_create’ [-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>