drm/i915: use msleep for intel_wait_for_vblank

20ms delay is quite big and the routine isn't called in atomic context.
better use msleep to let other tasks run. This can reduce cpu time used
by Xorg, so potentially boost boot.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>