drm/i915: Clear the dpll_hw_state when disabling a pipe

Clear the dpll_hw_state when we're about disable the pipe.
Previously it looks like we just left the old junk in there.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220325123205.22140-8-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>