drm/i915: add runtime PM support on Haswell

The code to enable/disable PC8 already takes care of saving and
restoring all the registers we need to save/restore, so do a put()
call when we enable PC8 and a get() call when we disable it.

Ideally, in order to make it easier to add runtime PM support to other
platforms, we should move some things from the PC8 code to the runtime
PM code, but let's do this later, since we can make Haswell work right
now.

V2: - Rebase

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Don't actually enable runtime pm since I didn't merge all
patches.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>