dma-buf: drop dynamic_mapping flag

Instead use the pin() callback to detect dynamic DMA-buf handling.
Since amdgpu is now migrated it doesn't make much sense to keep
the extra flag.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/353997/?series=73646&rev=1