drm/amd/display: Remove unused flag from stream state

[why & how]
Removing unused DSC flag which is incorrect and is not used.
We are only using stream->timing.flags.DSC for DSC's current
state. Stream state as an input parameter and should not contain
any past status flags.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Acked-by: Bindu Ramamurthy <bindur12@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>