intel: fix the wrong code to detect null texture.

intel: fix the wrong code to detect null texture.

There is already comments show how to detect a null texture. Fix the
code to match the comments.

This would fix the oglc divzero(basic.texQOrWEqualsZero) and
divzero(basic.texTrivialPrim) test case fail.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>