drm/bridge: analogix_dp: remove unused platform power_on_end callback

This isn't used, but gives the impression of the power on and power off
platform calls being non-symmetrical. Remove the unused callback and
rename the power_on_start to simply power_on.

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/20240619182200.3752465-1-l.stach@pengutronix.de