drm/i915: Extract p2 divider correctly for gen2 LVDS dual channel

In order to determine the correct p2 divider for LVDS on gen2,
we need to check the CLKB mode from the LVDS port register to
determine if we're dealing with single or dual channel LVDS.

Cc: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>