drm/amd/display: Remove dc_target object

dc_target does not fit well into DRM framework so removed it.
This will prevent the driver from leveraging the pipe-split
code for tiled displays, so will have to be handled at a higher
level.  Most places that used dc_target now directly use dc_stream
instead.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>