drm/radeon/kms: set DMA mask properly on newer PCI asics

If a card wasn't PCIE, we always set the DMA mask to 32 bits.
This is only applies to the old rage128/r1xx gart block on
early radeon asics (~r1xx-r4xx).  Newer PCI and IGP cards
can handle 40 bits just fine.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Chen Jie <chenj@lemote.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>