sunxi: video: Convert panel I2C to use DM_I2C

sunxi: video: Convert panel I2C to use DM_I2C

Two displays supported by the sunxi display driver (each one used by a
single board) require initialization over I2C. Both previously used
i2c_soft; replace this with the i2c-gpio instance that already exists in
those boards' device trees (sun5i-a13-utoo-p66 and sun6i-a31-colombus).

Since the i2c-gpio nodes are not referenced by any other node in the
device trees (the device trees have no panel node), the I2C bus is
selected by its node name.

This panel initialization code was the only i2c_soft user, so the
i2c_soft GPIO setup code can be removed now as well.

Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>