state_tracker: Fix check for scissor enabled when < 0.

DEQP's clear tests like to give us x + w < 0 or y + h < 0.  Since we
were comparing to an unsigned, it would get promoted to unsigned and come
out as bignum >= width or height and we would clear the whole fb instead
of none of the fb.

Fixes 10 tests under deqp-gles2/functional/color_clear.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>