i965/vec4/vp: Use vec4_visitor::CMP.

i965/vec4/vp: Use vec4_visitor::CMP.

... instead of emit(BRW_OPCODE_CMP, ...). In commit 6b3a301f I changed
vec4_visitor::CMP to set the destination's type to that of src0. In the
following commit (2335153f) I removed an apparently now unnecessary work
around for Gen8 that did the same thing.

But there was a single place that emitted a CMP instruction without
using the vec4_visitor::CMP function. Use it there.

And change dst_null_d to dst_null_f for good measure, since ARB vp
doesn't have integers.

Cc: "10.5" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89032
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>