drm/i915: access VLV regs through read/write switch

Since the offsets have all moved around.

v2: switch IS_DISPLAYREG and IS_VALLEYVIEW checks around since the latter is
    cheaper (Daniel)
    bail out early in IS_DISPLAYREG if the reg is in the new range (Daniel)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Fixup if cascading fail that broke HAS_FORCEWAKE machines.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>