i965: Add support for GL_ARB_buffer_storage.

i965: Add support for GL_ARB_buffer_storage.

It turns out we can allow COHERENT storage/mappings all the time,
regardless of LLC vs non-LLC.  It just means never using temporary
mappings to avoid GPU stalls, and on non-LLC we have to use the GTT intead
of CPU mappings.  If we were to use CPU maps on non-LLC (which might be
useful if apps end up using buffer_storage on PBO reads, to avoid WC read
slowness), those would be PERSISTENT but not COHERENT, but doing that
would require us driving the clflushes from userspace somehow.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>