radeonsi: Fix guardband computation for large render targets

Stop using 12.12 quantization for viewports that are not contained in
the lower 4k corner of the render target as the hardware needs to keep
both absolute and relative coordinates representable.

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Cc: 18.3 19.0 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 3c540e0a)