Commit https://aomedia-review.googlesource.com/c/aom/+/40541 replaced
a division in the DC intra predictor by an approximate
multiply+shift sequence.

Unfortunately, this approximation is able to produce out-of-range
values. For example, consider 4x8 DC_PRED, with bit depth = 10.
If all of the context pixels are 0x3FF (the max value), then we get:

sum = 12 * 0x3FF
expected_dc = (sum * 0xAB) >> 11 = 1024 = 0x400

This means that we need to insert a clip_pixel(_highbd) operation
at the end of the DC prediction, to bring this value back in range.

BUG=aomedia:1272

Change-Id: I9beb9ac8a4b39803865f7e23932402ecd1d6f672