drm/bridge: make devm_drm_bridge_alloc() mandatory for bridge allocation

All DRM bridges are now supposed to be allocated using
devm_drm_bridge_alloc(), which is cleaner and necessary to support
refcounting.

In the absence of a drm_bridge_init() or such initialization function,
document the new mandatory alloc function on the first DRM bridge core
function that is called after allocation, i.e. drm_bridge_add().

Suggested-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250326-drm-bridge-refcount-v9-3-5e0661fe1f84@bootlin.com


Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>