drm/amd/display: Change DSC policy from slices per column to minimum slice height

[why] Minimum slice height is recommended by VESA DSC Spreadsheet user guide

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>