drm/kms: Simplify setup of the initial I2C encoder config.

In most use cases the driver will be using the same static config all
the time: interpreting i2c_board_info::platform_data as the default
config we can can save the GPU driver a redundant set_config() call.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>