drm/i915: Remove extraneous MMIO for RPS

The values created at initialization must always exist to use the
interface. Reading them again is confusing, and pointless.

More cleanups are coming in the next patch. Since I am not 100% certain,
moreover on BYT, (though I am extremely close to that) that there is no
need to leave the MMIO here, I wanted to make it a separate patch for
the bisectable 'just-in-case'

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>