The construct allocating only parts of the vma structure when
the userptr part is not needed is very fragile. A developer could
add additional fields below the userptr part, and the code could
easily attempt to access the userptr part even if its not persent.

So introduce xe_userptr_vma which subclasses struct xe_vma the
proper way, and accordingly modify a couple of interfaces.
This should also help if adding userptr helpers to drm_gpuvm.

v2:
- Fix documentation of to_userptr_vma() (Matthew Brost)
- Fix allocation and freeing of vmas to clearer distinguish
  between the types.

Closes: https://lore.kernel.org/intel-xe/0c4cc1a7-f409-4597-b110-81f9e45d1ffe@embeddedor.com/T/#u
Fixes: a4cc60a5 ("drm/xe: Only alloc userptr part of xe_vma for userptrs")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240131091628.12318-1-thomas.hellstrom@linux.intel.com