Replace some commonly repeated code with a function.

v2: Use amdgpu_find_mm_node() in amdgpu_ttm_io_mem_pfn()
Signed-off-by: NHarish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>