drm/amdgpu: Make pin_size values atomic

Concurrent execution of the non-atomic arithmetic could result in
completely bogus values.

v2:
* Rebased on v2 of the previous patch

Cc: stable@vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/106872


Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>