drm/i915: remove rps local variables

With the renamed RPS struct members, it's easier to skip the local
variables which no longer clarify anything, and if anything just make
the code harder to read.

The real motivation for this patch is actually the next patch, which
attempts to consolidate some of the functionality.

Cc: Jeff McGee <jeff.mcgee@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>