drm/i915: Refactor gen6_set_rps

What used to be a short-circuit now needs to adjust interrupt masking in
response to user requests for changing the min/max allowed frequencies.
This is currently done by a special case and early return, but the next
patch adds another common action to take, so refactor the code to reduce
duplication.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>