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

commit fc6826d1dcd65f3d1e9a5377678882e4e08f02be upstream.

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>
[bwh: Backported to 3.2: adjust context; drop changes for Valley View]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>