llvmpipe: set max cube texture size to 4K x 4K

llvmpipe: set max cube texture size to 4K x 4K

Before, the limit was 8K.  For 32-bit RGBA that would be require 1.5 GB
of memory (w/out mipmaps).  That's well beyond the LP_MAX_TEXTURE_SIZE
of 1GB.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>