drm: xlnx: zynqmp_dpsub: Update dependencies for ZynqMP DP

ZynqMP DP requires the ZynqMP PHY and DPDMA to operate properly. So
depend on both the PHY and DPDMA.

Signed-off-by: Dylan Yip <dylan.yip@xilinx.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>