drm/amdgpu: enable AGP aperture on gmc10.x (v2)

Just a small optimization for accessing system pages directly.
Was missed for gmc v10 since the feature landed for older gmcs
while we were still on the emulator or gmc10 and we use the AGP
aperture for zfb on the emulator.

v2: fix up the system aperture as well

Reviewed-and-tested-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>