drm/bridge: imx8qxp-pixel-link: get/put the next bridge

drm/bridge: imx8qxp-pixel-link: get/put the next bridge

This driver obtains a bridge pointer from of_drm_find_bridge() in the probe
function and stores it until driver removal. of_drm_find_bridge() is
deprecated, so move to of_drm_find_and_get_bridge() for the bridge to be
refcounted and use bridge->next_bridge to put the reference on
deallocation.

To keep the code as simple and reliable as possible remove the intermediate
next_bridge and selected_bridge variables.

Get/put operations on the remaining pointer is pl->bridge.next_bridge,
which is tied to the struct imx8qxp_pixel_link lifetime, are:
 - get reference when assigned (by of_drm_find_and_get_bridge())
 - put reference before reassignment if reassignment happens
 - put reference when the struct imx8qxp_pixel_link embedding the
   struct drm_bridge is destroyed (struct drm_bridge::next_bridge)

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Acked-by: Liu Ying <victor.liu@nxp.com>
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Link: https://lore.kernel.org/r/20260211-drm-bridge-alloc-getput-drm_of_find_bridge-v6-1-651ddfd13bdb@bootlin.com