gpu: drm: gma500: Use vma_pages()

Replace explicit computation of vma page count by a call to
vma_pages()

Signed-off-by: Shyam Saini <mayhs11saini@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1476054436-9378-1-git-send-email-mayhs11saini@gmail.com