drm/i915: Only look for matching clocks for LVDS downclock

This patch enforces that the downclock clock source is the same as the preferred
clock source for LVDS. This fixes a bug where the driver chooses a downclock
clock source with a different P than the preferred mode clock source. This
happened even if the preferred clock source implemented an acceptable rate for
the downclock. The result of this bug is that downclock is disabled.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>