drm/amd/display: Add dsc pre-validation in atomic check

[Why]
The previous change:
"Add affected crtcs to atomic state for dsc mst unplug"
forces modeset on all added crctc regardless whether timing changed or not.
Per our implementation of dsc we need modeset only if timing changed.
Otherwise dsc can be programmed incorrectly leading to dsc engine hang.

[How]
During atomic_check pre-compute dsc params.
Only set mode_changed if timing is changed.

Reviewed-by: Hersen Wu <hersenwu@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Roman Li <Roman.Li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>