Don't calculate chroma data in monochrome mode

Encoder: Prior to this patch, some chroma data was calculated and
later discarded when in monochrome mode. This patch ensures that
the chroma planes are left uninitialised and that chroma
calculations are not performed.

Decoder: Prior to this patch, some chroma calculations were still
being performed in monochrome mode (e.g. loop filtering). This
patch ensures that calculations are only performed on the y
plane, with the chroma planes being set to a constant.

Change-Id: I394c0c9fc50f884e76a65e6131bd6598b8b21b10