Take into account whether or not the AGP aperture is
enabled or not when calculating the system aperture.

Fixes white screens with DCN 3.1.4.

Based on a patch from Yifan Zhang <yifan1.zhang@amd.com>

Cc: Yifan Zhang <yifan1.zhang@amd.com>
Acked-by: NHarry Wentland <harry.wentland@amd.com>
Reviewed-by: NYifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # 6.1.x