drm/amd/display: Minimize DSC resource re-assignment

[why]
Assigning a different DSC resource than the one previosly used is
currently not handled. This causes black screen on mode change when more
than one monitor is connected on some ASICs.

[how]
- Acquire the previously used DSC if available
- Make sure re-program is triggered if new DSC is used

Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>