drm/amd/display: Add a default case for dc_status_to_str

drm/amd/display: Add a default case for dc_status_to_str

[Why&How]
If a parsed dc_status case is not covered by the dc_status_to_str, the
switch case is skipped, and the function returns
"Unexpected status error".

This causes build failures when new dc_status enums are introduced.
Changing the 'return "Unexpected status error"' into default resolves it.

Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>