This commit adds an encoder workaround to support better
compatibility with a non-compliant hardware vp9 profile 2 decoder.

The known issue with this decoder is:
The decoder assumes a wrong value, 127 instead of the correct
value of 511 and 2047, for any assumed top-left corner pixel in
UV planes for 10 and 12 bit, respectively. Such assumed
top-left corner pixel is used for INTRA prediction when a real
decoded/reconstructed pixel is not avalable, e.g. when it is
located inside the row above the top row or inside the column
left to the leftest column of a video image.

Change-Id: Ic15a938a3107e1b85e96cb7903a5c4220986b99d