drm/amdgpu: Refactor code to handle non coherent and uncached

This simplifies existing coherence handling for Arcturus and Aldabaran
to account for !coherent && uncached scenarios.

Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>