drm/amdgpu: fix incorrect use of drm_file->pid

That's the PID of the creator of the file (usually the X server) and not
the end user of the file.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
CC: stable@vger.kernel.org