drm/bridge: adv7511: Rework adv7511_power_on/off() so they can be reused internally

In chasing down issues with EDID probing, I found some
duplicated but incomplete logic used to power the chip on and
off.

This patch refactors the adv7511_power_on/off functions, so
they can be used for internal needs.

Cc: David Airlie <airlied@linux.ie>
Cc: Archit Taneja <architt@codeaurora.org>
Cc: Wolfram Sang <wsa+renesas@sang-engineering.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1484614372-15342-5-git-send-email-john.stultz@linaro.org