MLK-16015-6 drm/imx: ldb: Add pixel link validate/invalidate logics

To enable or disable a display safely, we need to validate pixel link
after the relevant ldb channel is enabled and invalidate pixel link
before the channel is disabled.  These operations are recommended
by the design team.

Signed-off-by: Liu Ying <victor.liu@nxp.com>
(cherry picked from commit 5fab9a74)