radv: Consider maximum sample distances for entire grid.

The other pixels in the grid might have samples with a larger
distance than the (0,0) pixel.

Fixes dEQP-VK.pipeline.multisample.sample_locations_ext.verify_location.samples_8_packed
when CTS is compiled with clang.

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <mesa/mesa!4480>