media: i2c: adv748x: Handle TX[A|B] power management

As the driver is now allowed to probe with a single output endpoint,
power management routines shall now take into account the case a CSI-2 TX
is not enabled.

Unify the adv748x_tx_power() routine to handle transparently TXA and TXB,
and enable the CSI-2 outputs conditionally.

Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>