drm: xlnx: zynqmp_dpsub: Move all DRM init and cleanup to zynqmp_kms.c

Continue the isolation of DRM/KMS code by moving all DRM init and
cleanup from zynqmp_dpsub.c to zynqmp_kms.c.

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