drm/xlnx: Use devm_drm_dev_alloc

Gets rid of drmm_add_final_kfree, which I want to unexport so that it
stops confusion people about this transitional state of rolling drm
managed memory out.

This also fixes the missing drm_dev_put in the error path of the probe
code.

v2: Drop the misplaced drm_dev_put from zynqmp_dpsub_drm_init (all
other paths leaked on error, this should have been in
zynqmp_dpsub_probe), now that subsumed by the auto-cleanup of
devm_drm_dev_alloc.

Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Hyun Kwon <hyun.kwon@xilinx.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200907082225.150837-1-daniel.vetter@ffwll.ch