drm/msm/dpu: Add a null ptr check for dpu_encoder_needs_modeset

drm/msm/dpu: Add a null ptr check for dpu_encoder_needs_modeset

The drm_atomic_get_new_connector_state() can return NULL if the
connector is not part of the atomic state. Add a check to prevent
a NULL pointer dereference.

This follows the same pattern used in dpu_encoder_update_topology()
within the same file, which checks for NULL before using conn_state.

Signed-off-by: Chenyuan Yang <chenyuan0y@gmail.com>
Fixes: 1ce69c265a53 ("drm/msm/dpu: move resource allocation to CRTC")
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/665188/
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>