drm/msm/mdp5: fix incorrect parameter for msm_framebuffer_iova()

The index of ->planes[] array (3rd parameter) cannot be equal to MAX_PLANE.
This looks like a typo that is now fixed.

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>