Making lane stagger calculation common for HDMI and DP

v2: Imre's comments addressed
	- Remove lane stagger from bxt_clk_div and make it a local variable in
	ddi_pll_select
Signed-off-by: NVandana Kannan <vandana.kannan@intel.com>
Reviewed-by: NImre Deak <imre.deak@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>