drm/i915: add Punit read/write routines for VLV v2

Slightly different than other platforms.

v2 [Jani]: Fix IOSF_BYTE_ENABLES_SHIFT shift. Use common routine.
v3: drop turbo defines from this patch (Ville)
    use PCI_DEVFN(2,0) instead of open coding (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Add checkpatch bikeshed about missing space.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>