Revert "drm/i915: Don't overclock on Haswell"

Revert "drm/i915: Don't overclock on Haswell"

This reverts commit fec46b5eff854df5647a9f4724e45dd33933855a.

The latest version of our PM programming doc (which is WAY better than
previous versions, and thanks for that) says something along the lines
of, "On Haswell overclocking is no long achieved via mailbox registers."
Which I misinterpreted as, the driver must done something different than
it did on IVB, and SNB.

It appears I jumped the gun, and that's all false. We've gotten some
clarification, and it appears at least *reading* the overclocking
information works in exactly the same manner.

Cc: kim.l.saw-chu@intel.com
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>