drm/i915: Fix computation of last_adjustment for RPS autotuning

The issue is that by computing the last_adj value after applying the
clamping, we can end up with a bogus value for feeding into the next RPS
autotuning step.

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