drm/xe/bo: handle PL_TT -> PL_TT

When moving between PL_VRAM <-> PL_SYSTEM we have to have use PL_TT in
the middle as a temporary resource for the actual copy. In some GL
workloads it can be seen that once the resource has been moved to the
PL_TT we might have to bail out of the ttm_bo_validate(), before
finishing the final hop. If this happens the resource is left as
TTM_PL_FLAG_TEMPORARY, and when the ttm_bo_validate() is restarted the
current placement is always seen as incompatible, requiring us to
complete the move.  However if the BO allows PL_TT as a possible
placement we can end up attempting a PL_TT -> PL_TT move (like when
running out of VRAM) which leads to explosions in xe_bo_move(), like
triggering the XE_BUG_ON(!tile).

Going from TTM_PL_FLAG_TEMPORARY with PL_TT -> PL_VRAM should already
work as-is, so it looks like we only need to worry about PL_TT -> PL_TT
and it looks like we can just treat it as a dummy move, since no real
move is needed.

Reported-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>