drm/amd/display: fix dml handling of mono8/16 pixel formats

mono formats are treated exactly the same as equivallent bpp
444 formats. Dml validation however lacks 444 8 bit format
while dml perf param calculation lacks mono format support

This change makes them equivallent as far as the enum is concerned
to avoid having to update dml

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>