drm/i915: s/DPPL/DPLL/ for SKL DPLLs

SKL DPLLs shouldn't be called DPPLs.

Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Fixes: 2edd6443

 ("drm/i915: Use a table to initilize shared dplls")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462993473-8254-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>