drm/i915/vlv: add early DPIO init v3

Just add an early init since we may need to access DPIO regs early on.
The init call in modeset_init_hw is also needed for the resume case,
when we need to reset DPIO to keep things happy.

v2: split reset and reg init
v3: split patches (Daniel)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>