drm/i915: Fix mbus join config lookup

drm/i915: Fix mbus join config lookup

The bogus loop from compute_dbuf_slices() was copied into
check_mbus_joined() as well. So this lookup is wrong as well.
Fix it.

Cc: stable@vger.kernel.org
Fixes: f4dc00863226 ("drm/i915/adl_p: MBUS programming")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220207132700.481-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
(cherry picked from commit 053f2b85631316a9226f6340c1c0fd95634f7a5b)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>