drm/i915: add an explict mmio base for gpio/gmbus io

Again, Valleyview modes these around, so make the mmio base more
explicit to consolidate the base address computations to one
HAS_PCH_SPLIT check.

v2: Fix up the PCH_SPLIT braino ... it actually works that way round.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>