dma-buf & drm/amdgpu: remove dma_resv workaround

Rework the internals of the dma_resv object to allow adding more than one
write fence and remember for each fence what purpose it had.

This allows removing the workaround from amdgpu which used a container for
this instead.

Signed-off-by: Christian König <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Cc: [email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]