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

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97593Tested-by: NKonstantin A. Lepikhov <lakostis@altlinux.org>
Tested-by: NArek Ruśniak <arek.rusi@gmail.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>