drm/i915: Drop stray clearing of rps->last_adj

We used to reset last_adj to 0 on crossing a power domain boundary, to
slow down our rate of change. However, commit 60548c55 ("drm/i915:
Interactive RPS mode") accidentally caused it to be reset on every
frequency update, nerfing the fast response granted by the slow start
algorithm.

Fixes: 60548c55 ("drm/i915: Interactive RPS mode")
Testcase: igt/pm_rps/mix-max-config-loaded
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180802100631.31305-1-chris@chris-wilson.co.uk