drm/amdgpu: fix JPEG instance checking when ctx init

Use proper structure.

Fixes: 0388aee7 ("drm/amdgpu: use the JPEG structure for general driver support")
Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>