highpass_filter: Refactor the highbd and lowbd versions.
highpass_filter: Refactor the highbd and lowbd versions.

Almost the whole function was common except the type of 'dgd', so use a
common helper function.

BUG=aomedia:442

Change-Id: I4bf20dce5f2faeaa7e6255452282f7653090b990

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