drm: xilinx: connector: Configure the type from DT property

The connector can be either HDMI or DisplayPort depending on which encoder
slave is used. Add the struct xilinx_drm_connector_type to have supported
connector types, and decide the type based on DT.

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