gpu: drm: gma500: Use vma_pages()

Replace explicit computation of vma page count by a call to
vma_pages()
Signed-off-by: NShyam Saini <mayhs11saini@gmail.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1476054436-9378-1-git-send-email-mayhs11saini@gmail.com

drivers/gpu/drm/gma500/framebuffer.c

@@ -124,7 +124,7 @@ static int psbfb_vm_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 	unsigned long phys_addr = (unsigned long)dev_priv->stolen_base +
 				  psbfb->gtt->offset;
 
-	page_num = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
+	page_num = vma_pages(vma);
 	address = (unsigned long)vmf->virtual_address - (vmf->pgoff << PAGE_SHIFT);
 
 	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);