Avoid clobbering the registers q4-q7 in DeblockingBSCalcEnc_neon

Remap q5 to q8, q6 to q9, q7 to q10 and q8 to q11, and push
q4 to the stack.

This was missed previously since the codec unittest doesn't
test encoding with loop filter enabled yet.
Merge pull request #501 from mstorsjo/neon-register-backup

Avoid clobbering the registers q4-q7 in DeblockingBSCalcEnc_neon