drm/radeon/kms: enable unmappable vram for evergreen

Evergreen now has blit support, but unmappable vram support
was disabled in c919b371cb734f42b1130e706ecee262f8d9261d
(drm/radeon/kms: avoid corner case issue with unmappable vram V2)
due to merge ordering.  This re-enables unmappable vram on
evergreen.

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