drm/i915/bdw: Clock gating init

Clock gating init is really a catch all function for registers we need
to write early in loading the driver.

Atm just the bare metal stuff we need, more will surely come.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>