Fix compile warning with mono-video disabled
Fix compile warning with mono-video disabled

The variable 'num_planes' is only used when mono-video is enabled,
so move it inside a #if CONFIG_MONO_VIDEO block

Change-Id: I415f764b2629478edde579142b7242851991b1c0

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom