drm/amd/display: set default return value for ODM Combine debugfs

[Why&How]
Set a default return value of -ENOTSUPP to indicate that the hardware
does not support querying ODM Combine mode.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>