drm: xilinx: encoder: Initialize the drm encoder before the encoder slave

drm_encoder_init() and drm_encoder_helper_add() need to be called before
initializing the encoder slave. The encoder slave can generate interrupts
right after initalization, which may result in error from not initialized
drm device.

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