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

This reverts commit 2cfe34e1.
Enable AGP aperture cause SDMA page fault for gfx11.0.2,
so temp disable AGP aperture until SDMA FW resolved this.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Jack Gui <Jack.Gui@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>