llvmpipe: fix using wrong format with MRT in blend code

llvmpipe: fix using wrong format with MRT in blend code

We were passing in the rt index however this was always 0 for non-independent
blend case. (The format was only actually used to decide if the color mask
covered all channels so this went unnoticed and was discovered by accident.)
Additionally, there was a second problem because we do fixups in the key based
on color buffer format we cannot use non-independent blend anyway as the fixed
up values would never get used.
So always turn non-independent blending into independent.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>