drm/bridge: imx8qxp-pixel-combiner: convert to devm_drm_bridge_alloc() API

This is the new API for allocating DRM bridges.

This driver embeds an array of channels in the main struct, and each
channel embeds a drm_bridge. This prevents dynamic, refcount-based
deallocation of the bridges.

To make the new, dynamic bridge allocation possible:

 * change the array of channels into an array of channel pointers
 * allocate each channel using devm_drm_bridge_alloc()
 * adapt the code wherever using the channels
 * remove the is_available flag, now "ch != NULL" is equivalent

Reviewed-by: Liu Ying <victor.liu@nxp.com>
Link: https://lore.kernel.org/r/20250509-drm-bridge-convert-to-alloc-api-v3-18-b8bc1f16d7aa@bootlin.com


Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>