drm/i915/dsi: clean up vlv gpio table and definitions

Define and store the pad base offset in the array, and reference the
pconf0 and padval registers through macros. Add VLV prefixes to
macros. Use spec nomenclature for pconf0 and padval.

v2: Address Ville's review comments, squash another patch here.

v3: Use the names Ville dug up in the specs.

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/34932140b78a3de7f825c78380a08c930694651b.1459884518.git.jani.nikula@intel.com