mesa: Fix double-lock of Shared->FrameBuffers and usage of wrong mutex
Fixes: 7534c536 Fixes: 8cfb3e4e Closes: mesa/mesa#3024 Signed-off-by:Danylo Piliaiev <danylo.piliaiev@globallogic.com> Reviewed-by:
Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com> Part-of: <mesa/mesa!5160> (cherry picked from commit 40255831)