drm: xilinx: drv: Iterate the connector list for suspend/resume

There can be multiple connectors. Iterate connector list to suspend
and resume. Store / restore the connector DPMS so that the connector
is in the correct state after resume. Remove the encoder/connector
pointers from struct xilinx_drm_private as they are no long needed.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>