nv50/ir: use round toward 0 when converting doubles to integers

nv50/ir: use round toward 0 when converting doubles to integers

Like floats, we should use the round toward 0 mode instead of the
nearest one (which is the default) for doubles to integers.

This fixes all arb_gpu_shader_fp64 piglits which convert doubles to
integers (16 tests).

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "11.2 12.0" <mesa-stable@lists.freedesktop.org>