drm/amd/display: Build DSC without DCN config

[Why & How]
DSC needs DCN but we are trying reduce the usage of the
DCN flag. It's easier to build the DSC code sprinkled
around DC core and the DMs and just guard the place where
DSC uses floating point code.

Since DSC is never enabled on non-DCN ASICs this won't
have any effect.

Reviewed-by: Hamza Mahfooz <Hamza.Mahfooz@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>