drm: xilinx: encoder: Remove dependency on adv7511

The previous adv7511 patch moves the dependency code to inside the adv7511
driver. This makes the xilinx encoder more generic to have other encoders.
Do not call encoder_sfuncs->set_config() when a mode is set, as the adv7511
driver handles it internally.

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