i965/tex: Don't create read-write textures with non-renderable formats

i965/tex: Don't create read-write textures with non-renderable formats

I haven't actually seen this bug in the wild, but it's possible that
someone could ask to do a S3TC PBO download or something.  This protects us
from accidentally creating a render target with a compressed or otherwise
non-renderable format.

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