drm/i915: Convert straggling MCHBAR registers

All our registers which are written through the MCHBAR are defined
descriptively as an offset to the MCHBAR. We had 3 outliers here.
Convert these as well so all registers which are offsets are MCHBAR can
be easily identified/found within the code.

With this, convert DCLK to also follow this format.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>