drm/xe: Fix xe_vma_set_pte_size

xe_vma_set_pte_size had a return value and did not set the 4k VMA flag.
Both of these were incorrect. Fix these.

Fixes: c47794bd ("drm/xe: Set max pte size when skipping rebinds")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240219211942.3633795-2-matthew.brost@intel.com