radeonsi: fix assertion failure by using the correct type

src/gallium/drivers/radeonsi/si_state_viewport.c:196: si_emit_guardband:
Assertion `vp_as_scissor.maxx <= max_viewport_size[vp_as_scissor.quant_mode]
&& vp_as_scissor.maxy <= max_viewport_size[vp_as_scissor.quant_mode]' failed.

The comparison was unsigned, so negative maxx or maxy would fail.

Fixes: 3c540e0a "radeonsi: Fix guardband computation for large render targets"
(cherry picked from commit 3ad2a9b3)