drm/amdgpu: clean up memory/GDS/GWS/OA alignment code

- move all adjustments into one place
- specify GDS/GWS/OA alignment in basic units of the heaps
- it looks like GDS alignment was 1 instead of 4

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>