drm/amdgpu/jpeg: fix race condition issue for jpeg start

Fix race condition issue when multiple jpeg starts are called.

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