drm/amd/display: Return a correct error value

[why]
The function is expected to return instance of the timing generator
therefore we shouldn't be returning boolean in integer function,
and we shouldn't be returning zero so changing it to -1.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>