drm/bridge: Document bridge init order with pre_enable_prev_first

In order to satisfy the MIPI DSI initialization sequence the bridge
init order has been altered with the help of pre_enable_prev_first
in pre_enable and post_disable bridge operations.

Document the affected bridge init order with an example on the
bridge operations helpers.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230328170752.1102347-2-jagan@amarulasolutions.com