drm/msm/dpu: reorder pointer operations after sanity checks to avoid NULL deref

drm/msm/dpu: reorder pointer operations after sanity checks to avoid NULL deref

_dpu_encoder_trigger_start dereferences "struct dpu_encoder_phys *phys"
before the sanity checks which can lead to a NULL pointer dereference if
phys is NULL.

Fix this by reordering the dereference after the sanity checks.

Fixes: 8144d17a81d9 ("drm/msm/dpu: Skip trigger flush and start for CWB")
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Qasim Ijaz <qasdev00@gmail.com>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/647536/
Link: https://lore.kernel.org/r/20250408172223.10827-1-qasdev00@gmail.com
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>