drm/radeon/kms: MC vram map needs to be >= pci aperture size

The vram map in the radeon memory controller needs to be
>= the pci aperture size.  Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=28402



The problematic cards in the above bug have 64 MB of vram,
but the pci aperture is 128 MB and the MC vram map was only
64 MB.  This can lead to hangs.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>