Generalize rawclk handling by storing it in dev_priv.

Presumably our hrawclk readout works at least for CTG and ELK
since we've been using it for DP AUX on those platforms. There
are no real docs anymore after configdb vanished, so the only
reference is the public CTG GMCH spec. What bits are listed in
that doc match our code. The ELK GMCH spec have no relevant
details unfortunately.

The PNV situation is less clear. Starting from
commit aa17cdb4 ("drm/i915: initialize backlight max from VBT")
we assume that the CTG/ELK hrawclk readout works for PNV as well.
At least the results *seem* reasonable for one PNV machine (Lenovo
Ideapad S10-3t). Sadly the PNV GMCH spec doesn't have the goods on
the relevant register either.

So let's keep assuming it works for PNV,ELK,CTG and read it out on
those platforms. G33 also has hrawclk according to some notes
in BSpec, but we don't actually need it for anything, so let's not
even try to read it out there.

v2: Rebase due to IS_VALLYVIEW vs. IS_CHERRYVIEW split
    Use KHz() all over, and kill off a few useless temp variables
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456932138-14004-2-git-send-email-ville.syrjala@linux.intel.comReviewed-by: NJani Nikula <jani.nikula@intel.com>