drm: encoder_slave: use new I2C API

i2c_new_client() is deprecated, use the replacement
i2c_new_client_device(). Also, we have a helper to check if a driver is
bound. Use it to simplify the code. Note that this changes the errno for
a failed device creation from ENOMEM to ENODEV. No callers currently
interpret this errno, though, so we use this condensed error check.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200316163907.13709-3-wsa+renesas@sang-engineering.com