drm: platform encoder driver support

Add struct drm_platform_encoder_driver. This structure can be used for
the encoder slave drivers on a platform bus. The i2c specific assumption
better be removed, and other drm encoder slave interfaces would be updated
to be more generic.

Signed-off-by: Hyun Kwon <hyunk@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>