drm/bridge: anx7625: refactor power control to use runtime PM framework

The driver originally use an atomic_t for keep track of the power
status, which makes the driver more complicated than needed, and has
some race condition as it's possible to have the power on and power off
sequence going at the same time.

This patch remove the usage of the atomic_t power_status, and use the
kernel runtime power management framework instead.

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Reviewed-by: Tzung-Bi Shih <tzungbi@google.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210517063553.554955-1-pihsun@chromium.org