Clean up 4x4 coefficient decoding code.
Don't use vp9_decode_coefs_4x4() for 2nd order DC or luma blocks. The code introduces some overhead which is unnecessary for these cases. Also, remove variable declarations that are only used once, remove magic offsets into the coefficient buffer (use xd->block[i].qcoeff instead of xd->qcoeff + magic_offset), and fix a few Google Style Guide violations. Change-Id: I0ae653fd80ca7f1e4bccd87ecef95ddfff8f28b4