drm/amd/display: config to override DSC start slice height

[why]
It's sometimes useful to have this option when debugging

[how]
Add a config flag. If the flag is not set, use driver default policy.
If the flag is set, use the value from the flag as the starting DSC slice
height.

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