drm/amd/display: Update drm_dsc to reflect native 4.2.0 DSC spec

[Why]
Some parts of the DSC spec relating to 4.2.0 were not reflected in
drm_dsc_compute_rc_parameters, causing unexpected config failures

[How]
Add nsl_bpg_offset and rbs_min computation

Signed-off-by: David Francis <David.Francis@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>