mesa: fix mismatch when returning 64-bit bindless uniform handles

mesa: fix mismatch when returning 64-bit bindless uniform handles

The slower convert-and-copy process performs a bad conversion
because it converts the value to signed 64-bit integer, but
bindless uniform handles are considered unsigned 64-bit.

This fixes "Check glUniform*() with mixed texture units/handles"
from arb_bindless_texture-uniform piglit.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Cc: "17.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>