llvmpipe: simplify buffer allocation logic.

llvmpipe: simplify buffer allocation logic.

Now with buffer formats clarification don't need all that logic any longer.
(Note that it never would have worked in any case, because blockwidth and
blockheight were swapped any allocation with multi-byte format would have
had zero size.)

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