drm/rockchip: dw_dp: Switch to drmm_kzalloc()

In the Linux kernel, the following vulnerability has been resolved: drm/rockchip: dw_dp: Switch to drmm_kzalloc() Driver makes use of drmm_encoder_init() to initialize the encoder and automatically handle the cleanup by registering drm_encoder_cleanup() with drmm_add_action(). However, the internal structure containing the encoder part gets allocated with devm_kzalloc(), which happens while component_bind_all() is being called from Rockchip DRM driver. The component framework further ensures it is deallocated as part of releasing all the resources claimed during bind, which is triggered from component_unbind_all(). When the reference to the DRM device gets eventually dropped via drm_dev_put() in rockchip_drm_unbind(), drmm_encoder_alloc_release() attempts to access the now released encoder structure, leading to use-after-free. Ensure driver's internal structure is still reachable on encoder cleanup by switching from a device-managed allocation to a drm-managed one.