CHROMIUM: gpu: i915: optimize vblank timeout

CHROMIUM: gpu: i915: optimize vblank timeout

On the resume path, wait_for_vblank is called 4 times and waits until its
timeout. To reduce the delays on that path, let's compute the maximum vblank
delay instead of always using the arbitrary 50 ms value.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
BUG=chrome-os-partner:13364
TEST=On Link, measure resume time and see a 130ms improvement.

Change-Id: I88231de43573432b6d71255f622eb9860fe3c28d
Reviewed-on: https://gerrit.chromium.org/gerrit/34624
Reviewed-by: Sameer Nanda <snanda@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>