drm/bridge: analogix_dp: register AUX bus after enabling runtime PM

AUX transactions require the controller to be in working state and
take a runtime PM reference. To avoid potential races beween the
first transactions on the bus and runtime PM being set up, move the
AUX registration behind the runtime PM setup.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240619182200.3752465-3-l.stach@pengutronix.de