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: d7d0e73 ("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>

drivers/gpu/drm/msm/disp/dpu1/dpu_encoder_phys_wb.c

@@ -252,11 +252,6 @@ static int dpu_encoder_phys_wb_atomic_check(
 	DPU_DEBUG("[atomic_check:%d, \"%s\",%d,%d]\n",
 			phys_enc->wb_idx, mode->name, mode->hdisplay, mode->vdisplay);
 
-	if (!conn_state->writeback_job || !conn_state->writeback_job->fb)
-		return 0;
-
-	fb = conn_state->writeback_job->fb;
-
 	if (!conn_state || !conn_state->connector) {
 		DPU_ERROR("invalid connector state\n");
 		return -EINVAL;
@@ -267,6 +262,11 @@ static int dpu_encoder_phys_wb_atomic_check(
 		return -EINVAL;
 	}
 
+	if (!conn_state->writeback_job || !conn_state->writeback_job->fb)
+		return 0;
+
+	fb = conn_state->writeback_job->fb;
+
 	DPU_DEBUG("[fb_id:%u][fb:%u,%u]\n", fb->base.id,
 			fb->width, fb->height);