MLK-18995 drm/imx: hdp: Set pixel link vld and sync ctrl in enc->enable/disable

MLK-18995 drm/imx: hdp: Set pixel link vld and sync ctrl in enc->enable/disable

The pixel link validation and sync ctrl enablement should be the last
step to enable the display pipeline which involves the HDP encoder.
So, let's move the pixel link operations from initialization stage to
enc->enable/disable.  Also, the pixel_link_init/deinit hooks are replaced
with pixel_link_validate/invalidate and pixel_link_sync_ctrl_enable/disable,
since the display controller driver has already initialized the pixel link
at the driver probe stage.

Signed-off-by: Liu Ying <victor.liu@nxp.com>
(cherry picked from commit cb234bc95e870a540e2f3a608f743a7e72a3e754)