drm/i915: Add additional pipe parameter for vlv_dpio_read and vlv_dpio_write. v2

The patch doesn't contain functional change, but is to prepare for
future platform which has different DPIO phy.  The additional pipe
parameter will use to select which phy to target for.

v2: Update the commit message and add static for the new function.
(Jani/Ville)

Signed-off-by: Chon Ming Lee <chon.ming.lee@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>