drm/i915/gt: Leave rps->cur_freq on unpark

Don't override our previous frequency we used after parking, and avoid
continually spiking back to the efficient frequency for mostly idle
workloads. Trust our ability to autotune across a workload switch.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200322163225.28791-2-chris@chris-wilson.co.uk