drm/i915: Refactor the deferred PM_IIR handling into a single function

This function, along with the registers and deferred work hander, are
all shared with SandyBridge, IvyBridge and their variants. So remove the
duplicate code into a single function.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>