drm/i915: remove confusing GPIO vs PCH_GPIO

Instead of defining all registers twice, define just a PCH_GPIO_BASE
that has the same address as PCH_GPIO_A and use that to calculate all
the others. This also brings VLV and !HAS_GMCH_DISPLAY in line, doing
the same thing.

v2: Fix GMBUS registers to be relative to gpio base; create GPIO()
    macro to return a particular gpio address and move the enum out of
    i915_reg.h (suggested by Jani)

v3: Move base offset inside the GPIO() macro so the GMBUS defines don't
    actually need to be changed (suggested by Daniel/Ville)

v4: Move definition of i915_gpio to intel_display.h and remove
    GMBUS/GPIO handling from gvt since now they have their own
    defines.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180727193647.8639-3-lucas.demarchi@intel.com