etnaviv: fix shader register control with MSAA

etnaviv: fix shader register control with MSAA

Apparently MSAA doesn't only add another input, but it also increases
required temporaries by one. Simple programs where the register demand
is given by the number of inputs did work fine, while more complex ones,
where register demand is given by the number of temporaries exhibit
rendering issues without this fix.

Cc: 22.3 mesa-stable
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19582>