drm/amd/display: Refactor stream encoder for HW review

Move DCN1 implementation of stream encoder to new file (instead
of common dce_stream_encoder.c).
Cleanup code related to different implementation due to register
definition differences.

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>