virgl: Fix injection of double from const mov instruction
We only copy two components, we have to use the complete original source, and we should rewrite the new source from scratch to avoid incorrect dimension and indirect handling.
This fixes piglit failures with spec@arb_gpu_shader_fp64@uniform_buffers@*
Fixes: 036d7172 (virgl: Move double operands to a temp to avoid double-swizzling bugs)