drm/i915: Use .disable_clock() for pll sanitation

Instead of every new platform having yet another masive
copy of the whole PLL sanitation code, let's just reuse the
.disable_clock() hook for this purpose. We do need to plug
this into the ICL+ DSI code for that, but fortunately it
already has a suitable function we can use.

We do lose the debug message though on account of not bothering
to check if the clock is actually enabled or not before turning
it off. We could introduce yet another vfunc to query the current
state, but not sure it's worth the hassle?

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210205214634.19341-14-ville.syrjala@linux.intel.com