drm/amdgpu: pre-map device buffer as cached for A+A config

For A+A configuration, device memory is supposed to be mapped as
cachable from CPU side. For kernel pre-map gpu device memory using
ioremap_cache

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Christian Koenig <Christian.Koenig@amd.com>
Tested-by: Amber Lin <Amber.Lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>