drm/bridge: drm_bridge_get/put(): document NULL pointer behaviour

drm/bridge: drm_bridge_get/put(): document NULL pointer behaviour

drm_bridge_get and drm_bridge_put() do nothing when they are passed a NULL
pointer, and they do so since their initial addition in commit 30d1b37d4c02
("drm/bridge: add support for refcounting").

This allows simpler code in various places when using these
functions. However it's not documented, so it's not clear whether it is
part of the API "contract" or just a current implementation detail that
might change in the future.

There is no visible reason to remove this NULL check, so document it,
making it part of the contract, letting users count on it.

Reviewed-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://patch.msgid.link/20260324-drm-bridge-alloc-getput-document-null-check-v1-1-fb0877c49d7e@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>