Because the mbmi pointer passed into is_smooth comes from the above/left
block, it might be an inter block. If this happens, we correctly deduce
that the above/left block does not use a smooth intra mode.

However, inter blocks do not set mbmi->uv_mode, so in the UV case we end up
reading stale data. This may result in is_smooth() returning the wrong value,
if (whatever was previously written into) mbmi->uv_mode happens to be a
smooth intra mode.

Fix this by including an explicit check for inter blocks.

BUG=aomedia:1362

Change-Id: I3ec9faef9b6297e22915176067b5704003bc4664