drm/i915: Restore the DPLL calculation logic for 9xx platform

The DPLL calculation logic for 9xx platform is changed in:
commit 652c393a
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Mon Aug 17 13:31:43 2009 -0700

    drm/i915: add dynamic clock frequency control

Maybe we will get the different M/N/P combination with that by using the
previous dpll calculation logic.

So restore the DPLL calculation logic for 9xx platform.

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