drm/ttm: let struct ttm_device_funcs be placed in rodata

Make the struct ttm_device_funcs pointers const so the data can be placed in rodata.

Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230309123700.528641-1-jani.nikula@intel.com