drm/xe/pt: check no-DMA huge-pte cases before DMA segment test

drm/xe/pt: check no-DMA huge-pte cases before DMA segment test

On a non-range clear, curs.size is never set, so the segment test
(next - va_curs_start > curs->size) returns false for every level > 0
before the clear_pt short-circuit is reached. The clear then descends to
level 0 instead of forming a huge zero-leaf, wasting page tables and
risking -ENOMEM on unbind.

Move the null-VMA, purged-BO and clear_pt short-circuits above the
curs->size test. The bind path always sets curs.size, so it is unaffected.

v2
- Also set curs.size on the clear path so the cursor stays meaningful
during the walk. clear_pt is only reached with range == NULL, so assert
that invariant. (Matthew Brost)

Cc: Matthew Brost <matthew.brost@intel.com>
Fixes: 5b658b7e89c3 ("drm/xe: Clear scratch page on vm_bind")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260728055916.593707-2-himal.prasad.ghimiray@intel.com
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
(cherry picked from commit 04eeeb45cb61b8a3e9d785003457e550c920ba49)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>