drm/i915: Call kill_bigjoiner_slave() earlier

Let's do the kill_bigjoiner_slave() thing from
intel_bigjoiner_add_affected_crtcs() since it's related to
what we do there. This cleans up the logic in the
compute_config() loop a bit.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201124201156.17095-4-ville.syrjala@linux.intel.com


Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>