anv: Fix calculation of guardband clipping region.

The existing guardband region calculation was mixing up x/y_min with x/y_max in cmd_buffer_emit_viewport(), causing the calculated viewport area to always be an empty region. Luckily intel_calculate_guardband_size() returns a non-empty but bogus guardband region in that case, so this doesn't seem to have led to conformance regressions, but the off-center guardbands could potentially impact performance in geometry-heavy rendering.

/cc @kwg, @gfxstrand