drm/amdgpu/jpeg: remove redundant check when it returns

Fix warning from kernel test robot
v2: remove the local variable as well

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>