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 <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220407085946.744568-4-christian.koenig@amd.com