As pointed out by rsbultje, my previous patch to is_smooth()
(a883e6ea) was not quite correct. This is because, when we're
making a chroma prediction, the uv_mode for the above/left chroma
predictions is not necessarily in above_mbmi/left_mbmi. Instead,
it may be in any of several places, depending on subsampling and
the values of mi_row/mi_col.

The cleanest solution is to explicitly maintain pointers to the
above and left chroma blocks. Then we can simply look at those
pointers when we want to know the above or left uv_mode.

Also include a bit of refactoring of get_filt_type: It seems
to be recalculating what's already in xd->{above,left}_mi,
so just use those directly.

BUG=aomedia:1362

Change-Id: I0230474a50d43b78cb587a2b553da9ca78cec0c6