drm/i915: Use i915_vma_offset() in intel_dpt_offset()

drm/i915: Use i915_vma_offset() in intel_dpt_offset()

Replace the open coded vma mm node stuff in intel_dpt_offset()
with i915_vma_offset(). This will also include the VT-d guard
in the result. Granted that should always be 0 for DPT, but
it seems prudent to include that in our DPT vma offset check
anyway.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250717171353.23090-5-ville.syrjala@linux.intel.com