drm/i915: Don't read out port_clock on CHV when DPLL is disabled

drm/i915: Don't read out port_clock on CHV when DPLL is disabled

Check whether the DPLL is even enabled before readoing out the dividers
and trying to derive port_clock on CHV. We already did this on VLV.

Also remove the comment "MIPI" comment from the VLV code since we call
this function whenever the pipe is enabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458052809-23426-9-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>