drm/i915/display: Fix shared dpll mismatch for bigjoiner slave

Currently when we do the HW state readout, we dont set the shared dpll to NULL
for the bigjoiner slave which should not have a DPLL assigned. So it has
some garbage while the HW state readout is NULL. So explicitly reset
the shared dpll for bigjoiner slave pipe.

Bug: https://gitlab.freedesktop.org/drm/intel/-/issues/3465


Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Tested-by: Swati Sharma <swati2.sharma@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714223414.9849-1-manasi.d.navare@intel.com