drm/bridge: Fix typo in post_disable() description

s/singals/signals/

Fixes: 199e4e96 ("drm: Extract drm_bridge.h")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231124094253.658064-1-dario.binacchi@amarulasolutions.com