drm/amd/display: fix mpo validation failure

There was an error in translation of mode support check.
"N/A" is a failure condition while "" was a special case.
This change will differentiate between the two by using a
define.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>