drm/i915/dp: Add a standalone function to obtain shared dpll for HSW/BDW/SKL/BXT

Add the PLL selection code for HSW/BDW/BXT/SKL into a stand-alone function
in order to allow for the implementation of a platform neutral upfront
link training function.

v4:
* Removed dereferencing NULL pointer in  case of failure (Dhinakaran Pandiyan)
v3:
* Add Hooks for all DDI platforms into this standalone function

v2:
* Change the macro to use dev_priv instead of dev (David Weinehall)

Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>