radeonsi: use better sample locations for 4x MSAA

radeonsi: use better sample locations for 4x MSAA

Discovered by luck. Verified with the piglit MSAA accuracy test.
It also shows that the worst case EQAA 16s4f results in very good 4x MSAA
in the worst case.

Nine might not like these positions, but they are prettier to the eye and
GL doesn't care.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>