drm/amd/display: Fix dml20v2_ModeSupportAndSystemConfigurationFull() to check DesiredBPP.

Why:
dml20v2_ModeSupportAndSystemConfigurationFull() didn't check against
DesiredBPP, so it doesn't work correctly when DesiredBPP can't be satisfied.

How:
Port the TruncToValidBPP() version from display_mode_vba_21.c to
display_mode_vba_20v2.c.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Bing Guo <bing.guo@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <anson.jacob@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>