dma-buf: fix unintended pin/unpin warnings

DMA-buf internal users call the pin/unpin functions without having a
dynamic attachment. Avoid the warning and backtrace in the logs.

Signed-off-by: Christian König <christian.koenig@amd.com>
Bugs: https://gitlab.freedesktop.org/drm/intel/-/issues/3481
Fixes: c545781e

 ("dma-buf: doc polish for pin/unpin")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
CC: stable@kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210517115705.2141-1-christian.koenig@amd.com