drm/amd/display: Drop to fail-safe mode if edid is bad

Provide the connector with a single fail-safe mode of 640x480 for CTS
tests instead of providing a list of possible base modes.

Signed-off-by: Mikita Lipski <mikita.lipski@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>