drm/i915: Remove unused function intel_ddi_get_link_dpll()

The function intel_ddi_get_link_dpll() was added in f169660e
("drm/i915/dp: Add a standalone function to obtain shared dpll for
HSW/BDW/SKL/BXT") to "allow for the implementation of a platform
neutral upfront link training function", but such implementation
never landed.

So remove that function and clean up the exported shared DPLL interface.

Fixes: f169660e ("drm/i915/dp: Add a standalone function to obtain shared dpll for HSW/BDW/SKL/BXT")
Cc: Durgadoss R <durgadoss.r@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Jim Bride <jim.bride@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484310032-1863-1-git-send-email-ander.conselvan.de.oliveira@intel.com