drm/i915: Use find_pll function to calculate DPLL setting for LVDS downclock

drm/i915: Use find_pll function to calculate DPLL setting for LVDS downclock

For any given clock we can use the find_pll to get the corresponding DPLL
setting. It is unnecessary to use the find_reduce_pll callback function
to calculate the DPLL parameter for LVDS downclock in order to get the same
divider factor(P) for the normal and downclock.

In theory when the LVDS downclock is supported by LVDS panel, we should get the
same DPLL divider factor(P) for the normal clock and reduced downclock.
If we get the diferent divider factor(P) for normal clock and reduced downclock,
it means that the found downclock is incorrect and should be discarded.

So we should use find_pll callback to calculate the DPLL parameter for the
LVDS reduced downclock as for the normal clock. Then we can do the cleanup
about find_reduced_pll.

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