drm/amd/display: Mark DSC resource as unused after copying to the secondary ODM pipe

[why]
DSC resource has to be acquired before it can be used and simply copying
a reference to it is very likely to cause problems when accessing DSC.

[how]
Set DSC resource pointer to NULL to mark it as unused after primary pipe
resources were copied to the secondary ODM pipe.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>