drm/amdgpu: Make pin_size values atomic

drm/amdgpu: Make pin_size values atomic

commit a5ccfe5c20740f2fbf00291490cdf8d2373ec255 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>