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

In the Linux kernel, the following vulnerability has been resolved:

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.

Patchwork: https://patchwork.freedesktop.org/patch/665188/