drm/amd/display: Clear encoder assignments when state cleared.

[Why]
State can be cleared without removing individual streams (by
calling dc_remove_stream_from_ctx()). This can leave the
encoder assignment module in an incoherent state and cause
future assignments to be incorrect.

[How]
Clear encoder assignments when committing 0 streams or
re-initializing hardware.

Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>