drm/msm: Don't break exclusive fence ordering

There's only one exclusive slot, and we must not break the ordering.

Adding a new exclusive fence drops all previous fences from the
dma_resv. To avoid violating the signalling order we err on the side of
over-synchronizing by waiting for the existing fences, even if
userspace asked us to ignore them.

A better fix would be to us a dma_fence_chain or _array like e.g.
amdgpu now uses, but
- msm has a synchronous dma_fence_wait for anything from another
  context, so doesn't seem to care much,
- and it probably makes sense to lift this into dma-resv.c code as a
  proper concept, so that drivers don't have to hack up their own
  solution each on their own.

v2: Improve commit message per Lucas' suggestion.

Cc: Lucas Stach <[email protected]>
Acked-by: Rob Clark <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Cc: Rob Clark <[email protected]>
Cc: Sean Paul <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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