drm/amd/display: return correct dc_status for dcn10_validate_global

[Why]
Before it was returning false in the case of failure even though return type should be enum dc_status

[How]
Return DC_FAIL_UNSUPPORTED_1 instead

Signed-off-by: Su Sung Chung <Su.Chung@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>