drm/xe/oa: Use vma_pages() helper function in xe_oa_mmap()

Use the vma_pages() helper function and remove the following
Coccinelle/coccicheck warning reported by vma_pages.cocci:

  WARNING: Consider using vma_pages helper on vma
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240819095751.539645-2-thorsten.blum@toblux.com

drivers/gpu/drm/xe/xe_oa.c

@@ -1244,8 +1244,7 @@ static int xe_oa_mmap(struct file *file, struct vm_area_struct *vma)
 	vm_flags_mod(vma, VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP | VM_DONTCOPY,
 		     VM_MAYWRITE | VM_MAYEXEC);
 
-	xe_assert(stream->oa->xe, bo->ttm.ttm->num_pages ==
-		  (vma->vm_end - vma->vm_start) >> PAGE_SHIFT);
+	xe_assert(stream->oa->xe, bo->ttm.ttm->num_pages == vma_pages(vma));
 	for (i = 0; i < bo->ttm.ttm->num_pages; i++) {
 		ret = remap_pfn_range(vma, start, page_to_pfn(bo->ttm.ttm->pages[i]),
 				      PAGE_SIZE, vma->vm_page_prot);