drm/amdgpu/gmc9: disable AGP aperture

We've had misc reports of random IOMMU page faults when
this is used.  It's just a rarely used optimization anyway, so
let's just disable it.  It can still be toggled via the
module parameter for testing.

v2: leave it configurable via module parameter

Reviewed-by: Yang Wang <kevinyang.wang@amd.com> (v1)
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Tested-by: Mario Limonciello <mario.limonciello@amd.com> # PHX & Navi33
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>