drm/radeon/kms: move r7xx/evergreen to its own vram_gtt setup function

MC_VM_FB_LOCATION is at a different offset between r6xx and r7xx/evergreen.
The location is needed for vram setup on fusion chips.

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