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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: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Yifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>