drm/amd/display: add dcn21 bw validation

[Why&How]
Create a separate dcn21_fast_validate_bw function for dcn21.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>