drm/i915: Use wait_for() to wait for Punit to change GPU freq on VLV

Use wait_for() instead of the open coded loop to avoid spreading the
same old timeout related bugs.

This changes the loop to use msleep(1) instead of udelay(10) when the
Punit had not yet completed the frequency change. In practice that
doesn't seem to hurt performance as the Punit appears to be ready pretty
much always.

Also give the status bit a name, instead of using the magic number 1.

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>