drm/i915/vlv: drop unused vlv_gps_core_read/write functions

Not needed.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/532150999335216b1374c606e1b3c253a6c9fe9d.1454582914.git.jani.nikula@intel.com