drm: zynq_drm: drm: Add power management

Add power management functions(suspend/resume). drm_helper_connector_dpms()
is used to suspend/resume the pipeline.

Signed-off-by: hyun woo kwon <hyunk@xilinx.com>
drm: zynq_drm: Use devm_kzalloc() instead of kzalloc()

Use a managed resource allocator, devm_kzalloc() instread of kzalloc()

Signed-off-by: hyun woo kwon <hyunk@xilinx.com>