drm: xlnx: zynqmp_dpsub: Move planes handling to zynqmp_kms.c

Decouple the planes handling from the display controller programming by
moving the corresponding code from zynqmp_disp.c to zynqmp_kms.c. This
prepares for using the DPSUB with a live video input, without creating
DRM planes in the DPSUB driver.

While at it, fix a typo in a comment.

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