drm/i915: Factor out p2 divider selection for pre-ilk platforms

The same dpll p2 divider selection is repeated three times in the
gen2-4 .find_dpll() functions. Factor it out.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>