[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: NNikola Cornij <nikola.cornij@amd.com>
Reviewed-by: NMartin Leung <Martin.Leung@amd.com>
Acked-by: NBhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>