drm/ttm: add ttm_bo_io_mem_pfn to check io_mem_pfn

The io_mem_pfn field was added in commit ea642c32 ("drm/ttm: add
io_mem_pfn callback") and is called unconditionally. However, not all
drivers were updated to set it.

Use the ttm_bo_default_io_mem_pfn function if a driver did not set its
own. And add new function ttm_bo_io_mem_pfn() as wrapper.

Signed-off-by: Michal Srb <msrb@suse.com>
Signed-off-by: Tan Xiaojun <tanxiaojun@huawei.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>