etnaviv: add mov for direct depth store output from load input

If the fragment shader writes fragment depth from an ALU instruction, the register allocator makes sure that the instruction is pointed to the correct register and write mask (t0._z). If there is no instruction emitted because the source is an input load intrinsic, or if the source instruction does not support swizzle and write mask, we have to add a mov instruction for this to work.

Fixes piglit test spec@glsl-1.10@execution@glsl-1.10-fragdepth.