drm/i915: add the BXT and CNL DPLL registers to pipe_config_compare

Looks like we were missing them.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170922205343.16006-2-paulo.r.zanoni@intel.com