gav1: Support monochrome videos/images

Support monochrome videos by filling U&V planes with neutral
values. Since all our output is in 8bit, the neutral value would
be 128.

Bug: 141654151
Test: AV1 related CTS tests continue to pass
Merged-In: I87eb5e11ab8bd88d4d3baf8d59c3bc53398b288d
Change-Id: I87eb5e11ab8bd88d4d3baf8d59c3bc53398b288d
(cherry picked from commit e7718810)