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