drm/{i915, xe}: Avoid direct inspection of dpt_vma from outside dpt

drm/{i915, xe}: Avoid direct inspection of dpt_vma from outside dpt

DPT code is so dependent on i915 vma implementation and it is not
ported yet to Xe.

This patch limits inspection to DPT's VMA struct to intel_dpt
component only, so the Xe GGTT code can evolve.

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240821193842.352557-4-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>