drm/msm/dpu: Fix pointer dereferenced before checking

drm/msm/dpu: Fix pointer dereferenced before checking

The phys_enc->wb_idx is dereferencing before null checking, so move
it after checking.

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Fixes: bceb6e7b768b7 ("drm/msm/dpu: introduce the dpu_encoder_phys_* for
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/487606/
Link: https://lore.kernel.org/r/1653877196-23114-1-git-send-email-baihaowen@meizu.com
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>