drm/i915: Move DP M/N setup from update_pll to mode_set for gmch platforms

There's no point in hiding the DP M/N setup in the update_pll functions.
Just move it to the mode_set function.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>