The max size was 16Kx16K so a 4 byte/pixel, six-sided cube would require
6 GBytes of memory. If mipmapped, 8 GB. Reduce the max size to 4K to
make the total size more reasonable.
Fixes a crash with the new piglit max-texture-size test.
Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>