dma-buf: begin/end_cpu might lock the dma_resv lock

At least amdgpu and i915 do, so lets just document this as the rule.

v2: Works better with less typos (intel-gfx-ci)

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20201214171622.3868883-1-daniel.vetter@ffwll.ch