vc4: Declare the cpu pointers as being modified in NEON asm.

vc4: Declare the cpu pointers as being modified in NEON asm.

Otherwise, the compiler is free to reuse the register containing the input
for another call and assume that the value hasn't been modified.  Fixes
crashes on texture upload/download with current gcc.

We now have to have a temporary for the cpu2 value, since outputs must be
lvalues.

(commit message by anholt)

Fixes: 4d30024238ef ("vc4: Use NEON to speed up utile loads on Pi2.")