radv: fix handling ViewportIndex with GPL

radv: fix handling ViewportIndex with GPL

When the fragment shader reads the viewport index as input and the last
pre-rasterization stage doesn't export it, it should be implicitly
zero (ie. first viewport). When the next stage is known, it's already
lowered in NIR, but with GPL we can't because we don't know if the FS
reads it. To fix this use AC_EXP_PARAM_DEFAULT_VAL_000.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18699>