drm/i915/bxt: add missing DDI PLL registers to the state checking

Although we have a fixed setting for the PLL9 and EBB4 registers, it
still makes sense to check them together with the rest of PLL registers.

While at it also remove a redundant comment about 10 bit clock enabling.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>