drm/arcpgu: remove drm_encoder_slave

drm_encoder_slave is the old way to write bridge drivers, for i2c
bridges only. It's deprecated, and definitely should not be used in
new drivers. This has absolutely nothing to do with the new bridge
driver infrastructure implemented by drm_bridge.

What's even strange is that arcpgu doesn't even use any of this, it
really only wants a plain normal drm_encoder. Nuke all the surplus
real estate.

v2: Actually git add after compile testing ...

v3: Clarify commit message and stop including drm_encoder_slave.h.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180117141755.16933-1-daniel.vetter@ffwll.ch