drm/i915: Factor out helper to read out PPS HW state

This will be needed by the next patch too so factor it out.

No functional change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466084243-5388-4-git-send-email-imre.deak@intel.com