drm/i915: Don't access fifodbg registers on gen8
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 <cd221c71e765d1ce593d15b204fee351fb09fd98@intel.com>
Reviewed-by: Ben Widawsky <73675debcd8a436be48ec22211dcf44fe0df0a64@bwidawsk.net> (v1)
Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs