drm/i915: Add support for detecting vblanks when hw frame counter is unavailable.

This uses the newly created drm_accurate_vblank_count_and_time to accurately
get a vblank count when the hw counter is unavailable.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463490484-19540-6-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>