drm/i915: Move the common RPS warnings to intel_set_rps()

Instead of having each back-end provide identical guards, just have a
singular set in intel_set_rps() to verify that the caller is obeying the
rules.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170220094713.22874-1-chris@chris-wilson.co.uk