drm/amd/display: pass the right num of modes added

[why]
In case if edid is null or corrupted we need to manually add
a single failsafe mode (640x480). If zero modes returned
DRM adds a different failsafe mode that is not accepted by
DP 1.2 compliance test

[how]
Return the number of modes manually added

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>