drm: xlnx: zynqmp_dpsub: Rename zynqmp_dpsub_handle_vblank with DRM prefix

The better convey its purpose, rename the zynqmp_dpsub_handle_vblank()
function that belongs to the DRM layer to
zynqmp_dpsub_drm_handle_vblank().

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