drm/amdgpu: add header file for macro SZ_1M

Fixes: 4dee6e4c

 ("drm/amdgpu: use linux size macro to simplify ONE_Kib & One_Mib")
Signed-off-by: Flora Cui <flora.cui@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>