drm/msm: Fix potential NULL dereference in DPU SSPP

Move initialization of sblk in _sspp_subblk_offset() after NULL check to
avoid potential NULL pointer dereference.

Fixes: 25fdd593 ("drm/msm: Add SDM845 DPU support")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jessica Zhang <jesszhan@codeaurora.org>
Link: https://lore.kernel.org/r/20211020175733.3379-1-jesszhan@codeaurora.org


Signed-off-by: Rob Clark <robdclark@chromium.org>