drm/doc/rfc: VM_BIND feature design document

VM_BIND design document with description of intended use cases.

v2: Reduce the scope to simple Mesa use case.
v3: Expand documentation on dma-resv usage, TLB flushing and
    execbuf3.
v4: Remove vm_bind tlb flush request support.
v5: Update TLB flushing documentation.
v6: Update out of order completion documentation.

Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220701003110.24843-2-niranjana.vishwanathapura@intel.com