drm/i915: Extend gpio read/write to other cores

Make the gpio read/write functions more generic iosf sideband read/write
functions, taking the iosf port as argument.

v2: rebase
v3: rebase
v4 by Jani: address Ville's review
v5 by Jani: drop the PCI_DEVFN change (Ville)

Signed-off-by: Deepak M <m.deepak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454604915-17142-1-git-send-email-jani.nikula@intel.com