drm: xilinx: Kconfig: Select DP subsystem driver for Xilinx DRM

The Xilinx DRM driver has dependency on DP subsystem driver, so select
DP subsystem driver for Xilinx DRM. This is just a quick solution as
there's no abstraction for subdevices in DRM framework, until all
subdevices are restructured with proper abstraction in Xilinx DRM.

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