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

Mark functions as static because they are not used outside the file
drm/ttm/ttm_bo_util.c.

This eliminates the following warnings in drm/ttm/ttm_bo_util.c:
drivers/gpu/drm/ttm/ttm_bo_util.c:190:5: warning: no previous prototype for ‘ttm_mem_reg_ioremap’ [-Wmissing-prototypes]
drivers/gpu/drm/ttm/ttm_bo_util.c:222:6: warning: no previous prototype for ‘ttm_mem_reg_iounmap’ [-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>