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Without this, we were only ever using the first 256MB of VRAM.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97593

Tested-by: Konstantin A. Lepikhov <lakostis@altlinux.org>
Tested-by: Arek Ruśniak <arek.rusi@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>