drm/xe: Fix overflow in vram manager

The overflow caused xe_bo_restore_kernel to return an error
Fix overflow in vram manager alloc function.

Signed-off-by: Riana Tauro <riana.tauro@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>