drm/i915: Split bxt_ddi_pll_select()

Split out of bxt_ddi_pll_select() the logic that calculates the pll
dividers and dpll_hw_state into a new function that doesn't depend on
crtc state. This will be used for enabling the port pll when doing
upfront link training.

v2:
* Refactored code so that bxt_clk_div need not be exported (Durga)
v1:
* Rebased on top of intel_dpll_mgr.c (Durga)
* Initial version from Ander on top of intel_ddi.c

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>