softpipe: set max cube texture size to 4Kx4K

softpipe: set max cube texture size to 4Kx4K

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>