drm/i915: Don't access fifodbg registers on gen8

as they don't exists.

v2: rename gen6_*_mt_* to gen7_*_mt_* as they never get called
    with gen6 (Chris)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>