drm/amd/display: remove legacy DSC functions

[why]
Clean up some dsc legacy functions that are
no longer needed.

[how]
remove two dsc functions in dc_dsc, use dc_bandwidth_in_kbps_from_timing
instead of calc_required_bandwidth_for_timing.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Nikola Cornij <Nikola.Cornij@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>