drm/i915: Streamline gpio_mmio_base deduction

drm/i915: Streamline gpio_mmio_base deduction

If we ignore the BXT situation, we can observe that the only variables
affecting gpio_mmio_base is IS_VALLEVIEW and HAS_GMCH_DISPLAY. The BXT
situation we can fit into the same pattern if we change gmbus_pins_bxt[]
to house the GMCH GPIO register offsets (like we do for all other
platfotms). So let's do that.

We could even simplify the VLV situation more by including the
display_mmio_offset in the GPIO register defines, but let's leave it be
for now.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446672017-24497-13-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>