drm/amdgpu/gfx: clean up harvest configuration (v2)

Read back harvest configuration from registers and simplify
calculations.  No need to program the raster config registers.
These are programmed as golden registers and the user mode
drivers program them as well.

v2: rebase on Tom's patches

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