st/xa: Fixups for PIPE_FORMAT_R8_UNORM A8 usage v2.
Check for PIPE_FORMAT_R8_UNORM when setting up the copy shader.
Also re-enable the dest alpha blending with A8 destination that
actually turned out to be correct.
Verified using rendercheck that the composite operators
overreverse, in, out, atop, atopreverse and xor seem to work fine
with a8 destiation.
v2: Fix a copy-paste error.
Reported-by: Jose Fonseca <jfonseca@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>