radeonsi: fix guardband handling for large values

radeonsi: fix guardband handling for large values

When minx = -32768 (ViewportBounds.Min) we'll hit the "left <= -1, ..."
assert because 'left' is computed as:

   (-65535 / 2 - translate_x) / (minx - translate_x)

This commit fixes the problem by using the full max_viewport_size => [-32768, 32767]
instead of [-32767, 32767] for SI_QUANT_MODE_16_8_FIXED_POINT_1_256TH.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3502
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6685>