drm/i915/bdw: Extract rp_state_caps logic

drm/i915/bdw: Extract rp_state_caps logic

We have a need for duplicated parsing of the RP_STATE_CAPS register (and
the setting of the associated fields). To reuse some code, we can
extract the function into a simple helper.

This patch also addresses the fact that we missed doing this for gen8,
something we should have done anyway.

This could be two patches, one to extract, and one to add gen8, but it's
trivial enough that I think one is fine. I will accept a request to
split it. Please notice the fix addressed by v2 below.

Valleyview is left untouched because it is different.

v2: Logically rebased on top of
commit d9e06ed691840402a1b65bced668b5137d2c4f53
Author: Jeff McGee <jeff.mcgee@intel.com>
Date:   Tue Feb 4 11:32:31 2014 -0600

    drm/i915: Restore rps/rc6 on reset

Note with the above change the fix for gen8 is also handled (which was
not the case in Jeff's original patch).

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