Revert "Revert "drm/amdgpu/gmc11: enable AGP aperture""

This reverts commit 1a65327a.

This should be resolved so we can re-enable this. Also,
the AGP apeture was bring programmed to 0 on MMHUB 3.0.1
since agp_start and end were not being set.

Acked-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Yifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>