drm/i915: Don't wait for Punit after each freq change on VLV

It seems that even though Punit reports the frequency change to have
been completed, it still reports the old frequency in the status
register for some time.

So rather than polling for Punit to complete the frequency change after
each request, poll before. This gets rid of the spurious "Punit overrode
GPU freq" messages.

This also lets us continue working while Punit is performing the actual
frequency change. As a result, openarena demo088-test1 timedemo average
fps is increased by ~5 fps, and the slowest frame duration is reduced
by ~25%.

The sysfs cur_freq file always reads the current frequency from Punit
anyway, so having rps.cur_delay be slightly off at times doesn't matter.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>