On ILK+ we current do a nuke right after activating FBC. If my
memory isn't playing tricks on me this is actially required if
FBC didn't stay disabled for a full frame. In that case the
deactivate+reactivate may not invalidate the cfb. I'd have to
double chekc to be sure though.

So let's keep the nuke, and just extend it backwards to cover
all the platforms by doing it a bit higher up.
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702204603.596-4-ville.syrjala@linux.intel.com