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>

drivers/gpu/drm/ttm/ttm_bo_util.c

@@ -187,7 +187,7 @@ void ttm_mem_io_free_vm(struct ttm_buffer_object *bo)
 	}
 }
 
-int ttm_mem_reg_ioremap(struct ttm_bo_device *bdev, struct ttm_mem_reg *mem,
+static int ttm_mem_reg_ioremap(struct ttm_bo_device *bdev, struct ttm_mem_reg *mem,
 			void **virtual)
 {
 	struct ttm_mem_type_manager *man = &bdev->man[mem->mem_type];
@@ -219,7 +219,7 @@ int ttm_mem_reg_ioremap(struct ttm_bo_device *bdev, struct ttm_mem_reg *mem,
 	return 0;
 }
 
-void ttm_mem_reg_iounmap(struct ttm_bo_device *bdev, struct ttm_mem_reg *mem,
+static void ttm_mem_reg_iounmap(struct ttm_bo_device *bdev, struct ttm_mem_reg *mem,
 			 void *virtual)
 {
 	struct ttm_mem_type_manager *man;