drm/amdgpu/gmc11: disable AGP on GC 11.5

AGP aperture is deprecated and no longer functional.

v2: fix typo (Alex)
v3: just skip the agp setup call
v4: revert back to the original model
v5: back to v3

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