drm/msm/dpu: Fix variable dereferenced before check

Fixes the following smatch warning:

drivers/gpu/drm/msm/disp/dpu1/dpu_encoder_phys_wb.c:261
dpu_encoder_phys_wb_atomic_check() warn: variable dereferenced before check 'conn_state'

Fixes: d7d0e73f

 ("drm/msm/dpu: introduce the dpu_encoder_phys_* for writeback")
Signed-off-by: sunliming <sunliming@kylinos.cn>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/490850/
Link: https://lore.kernel.org/r/20220623012707.453972-1-sunliming@kylinos.cn
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>