st/mesa: fix is_scissor_enabled when X/Y are negative

st/mesa: fix is_scissor_enabled when X/Y are negative

Similar to commit 49c24d8a24 ("i965: fix noop_scissor range issue on
width/height") - take the X/Y into account to determine whether the
scissor covers the whole area or not.

Fixes the recently-added gl-1.0-scissor-depth-clear-negative-xy piglit
test.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: <mesa-stable@lists.freedesktop.org>