drm/xe: Use BO's GT to determine dma_offset when programming PTEs

Rather than using the passed in GT, use the BO's GT determine dma_offset
when programming PTEs as these two GT's could differ (i.e. mapping a BO
from a remote GT). The BO's GT is correct GT to use as this where BO
resides, while the passed in GT is where the mapping is created.

v2:
  (Thomas)      - Kernel doc, extra new line
  (CI)          - Rebase to tip

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>