drm/amdgpu: fix a vcn4 boot poll bug in emulation mode

The return value should be set in vcn4 boot poll.

Signed-off-by: Sonny Jiang <sonny.jiang@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>