drm/xe: Perform dma_map when moving system buffer objects to TT

Currently we dma_map on ttm_tt population and dma_unmap when
the pages are released in ttm_tt unpopulate.

Strictly, the dma_map is not needed until the bo is moved to the
XE_PL_TT placement, so perform the dma_mapping on such moves
instead, and remove the dma_mappig when moving to XE_PL_SYSTEM.

This is desired for the upcoming shrinker series where shrinking
of a ttm_tt might fail. That would lead to an odd construct where
we first dma_unmap, then shrink and if shrinking fails dma_map
again. If dma_mapping instead is performed on move like this,
shrinking does not need to care at all about dma mapping.

Finally, where a ttm_tt is destroyed while bound to a different
memory type than XE_PL_SYSTEM, we keep the dma_unmap in
unpopulate().

v2:
- Don't accidently unmap the dma-buf's sgtable.

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/20240502183251.10170-1-thomas.hellstrom@linux.intel.com