llvmpipe: clamp scissors to be between 0 and max

llvmpipe: clamp scissors to be between 0 and max

We need to clamp to make sure invalid shader doesn't crash our
driver. The spec says to return 0-th index for everything that's
out of bounds.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: José Fonseca<jfonseca@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>