drm/ttm: remove ttm_bo_default_io_mem_pfn

No one will use this function except ttm_bo_io_mem_pfn() now, so move
the calculation of ttm_bo_default_io_mem_pfn() into ttm_bo_io_mem_pfn()
and do some cleanup.

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>