Currently we have the problem that the decision whether ptes need to
be (re)written is splattered all over the codebase. Move all that into
i915_vma_bind. This needs a few changes:
- Just reuse the PIN_* flags for i915_vma_bind and do the conversion
  to vma->bound in there to avoid duplicating the conversion code all
  over.
- We need to make binding for EXECBUF (i.e. pick aliasing ppgtt if
  around) explicit, add PIN_USER for that.
- Two callers want to update ptes, give them a PIN_UPDATE for that.

Of course we still want to avoid double-binding, but that should be
taken care of:
- A ppgtt vma will only ever see PIN_USER, so no issue with
  double-binding.
- A ggtt vma with aliasing ppgtt needs both types of binding, and we
  track that properly now.
- A ggtt vma without aliasing ppgtt could be bound twice. In the
  lower-level ->bind_vma functions hence unconditionally set
  GLOBAL_BIND when writing the ggtt ptes.

There's still a bit room for cleanup, but that's for follow-up
patches.

v2: Fixup fumbles.

v3: s/PIN_EXECBUF/PIN_USER/ for clearer meaning, suggested by Chris.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>