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

Mark function as static because it is not used outside file
drm/ttm/ttm_bo.c.

This eliminates the following warning in drm/ttm/ttm_bo.c:
drivers/gpu/drm/ttm/ttm_bo.c:960:5: warning: no previous prototype for ‘ttm_bo_move_buffer’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>