Add a motivation for and description of asynchronous VM_BIND operation

v2:
- Fix typos (Nirmoy Das)
- Improve the description of a memory fence (Oak Zeng)
- Add a reference to the document in the Xe RFC.
- Add pointers to sample uAPI suggestions
v3:
- Address review comments (Danilo Krummrich)
- Formatting fixes
v4:
- Address typos (Francois Dugast)
- Explain why in-fences are not allowed for VM_BIND operations for long-
  running workloads (Matthew Brost)
v5:
- More typo- and style fixing
- Further clarify the implications of disallowing in-fences for VM_BIND
  operations for long-running workloads (Matthew Brost)
v6:
- Point out that a gpu_vm is a virtual GPU Address space.
  (Danilo Krummrich)
- For an explanation of dma-fences point to the dma-fence documentation.
  (Paulo Zanoni)
- Clarify that VM_BIND errors are reported synchronously. (Paulo Zanoni)
- Use an rst doc reference when pointing to the async vm_bind document
  from the xe merge plan.
- Add the VM_BIND documentation to the drm documentation table-of-content,
  using an intermediate "Misc DRM driver uAPI- and feature implementation
  guidelines"
v7:
- Update the error handling documentation to remove the VM error state.
v8:
- Clarify error handling and difference in operation support between
  async VM_BIND and sync VM_BIND. (Paulo Zanoni)
- Update the sample uAPI with a self-contained example. (Paulo Zanoni)

Cc: Paulo R Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231012132552.20196-1-thomas.hellstrom@linux.intel.com