drm/xe/uapi: Be more specific about the vm_bind prefetch region

Let's bring a bit of clarity on this 'region' field that is
part of vm_bind operation struct. Rename and document to make
it more than obvious that it is a region instance and not a
mask and also that it should only be used with the prefetch
operation itself.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>