drm: xlnx: zynqmp_dpsub: Support operation without DMA engine

To prepare for usage of the DPSUB as a DisplayPort bridge without
creating a DRM device, make initialization and usage of the DMA engine
optional. The flag that controls this feature is currently hardcoded to
operating with the DMA engine, this will be made dynamic based on the
device tree configuration in a subsequent change.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>