drm: adv7511: Configure adv7511 with default values

This patch adds adv7511_set_default_config() which configures adv7511 device
with default values. This configuration can be overwritten with custom values
when client driver calls adv7511_set_config() explicitly.

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