freedreno/ir3: Fix loading half-float immediate vectors

freedreno/ir3: Fix loading half-float immediate vectors

Previously the code to load from a constant instruction was always
using the u32 pointer. If the constant is actually a 16-bit source
this would end up with the wrong values because the pointer would be
offset by the wrong size. This fixes it to use the u16 pointer.

Signed-off-by: Rob Clark <robdclark@chromium.org>