drm/i915/bxt: Move around lane stagger calculation

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: Vandana Kannan <vandana.kannan@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>