drm/amdkfd: fix a resource leakage issue

The function kfd_lookup_process_by_pasid will increase the reference
count of kfd_process object, its caller should call kfd_unref_process to
decrease the reference count. Otherwise resource leakage will happen.

Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>