drm/i915: Check whether the LVDS downclock is found in VBT

Enumerate the LVDS panel timing info entry list in VBT to check whether
the LVDS downclock is found. If found, the downclock is also used to switch
dynamically between low and high frequency for LVDS.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>