drm/vkms: Add atomic_helper_check_plane_state

Call atomic_helper_check_plane_state to clip plane coordinates.

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/b8c1bfed8b5720cc4794ace41cf49af66c99a48c.1532446182.git.hamohammed.sa@gmail.com