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