drm/i915: De-magic the VLV p2 divider step size

The p2 divider on VLV needs to be even when it's > 10. The current code
to make that happen is rather weird. Just make the step size adjustement
in the for loop decrement step.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>