drm/i915: Move the dpll_hw_state clearing to intel_dpll_crtc_compute_clock()

All .crtc_compute_clock() implementations do the same memset() to
clear the dpll_hw_state (since we preserve it across
intel_crtc_prepare_cleared_state()). Move the memset() to the common
wrapper.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220325123205.22140-7-ville.syrjala@linux.intel.com


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