drm: xlnx: zynqmp_dpsub: Manage DP and DISP allocations manually

The zynqmp_disp and zynqmp_dp structures are allocated with
drmm_kzalloc(). While this simplifies management of memory, it requires
a DRM device, which will not be available at probe time when the DP
bridge will be used standalone, with a DRM device in the PL. To prepare
for this, switch to manual allocation for zynqmp_disp and zynqmp_dp. The
cleanup still uses the DRM managed infrastructure, but one level up, at
the top level. This will be addressed separately.

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