nir: fix lower_wpos_ytransform in load_frag_coord case

nir: fix lower_wpos_ytransform in load_frag_coord case

Apparently we never hit this path.  Or at least haven't for a rather
long time.  But in either case (load_deref or load_frag_coord), we can
just directly use the intrinsic's ssa dest.  So stop passing the
nir_variable (which would be NULL in the load_frag_coord case) around
and instead just use &intr->dest.ssa.

(This ofc means we need to setup the cursor to insert *after* the
instruction, which seems to be another bug of the original
implementation.)

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>