Restore vp9_asm_enc_offsets.c

The previous commit 15255eef "Move dequant from BLOCKD to per-plane MACROBLOCKD"
removed the vp9_asm_enc_offsets.c file, but didn't update the various secondary
build systems that special case these files. Restore it for now, to ensure any
in-progress changes and builds continue working, to allow time to more carefully
coordinate removal of these files.

Change-Id: I24b78db3fb874d5fbd226548b7366a05ed98e536