r300g: don't set sample positions to the pixel center if MSAA is disabled

r300g: don't set sample positions to the pixel center if MSAA is disabled

but an MSAA resource is bound. This effectively makes the MSAA disable switch
not affect rasterization, but it still affects the alpha-to-one and
alpha-to-coverage states. This hardware just lacks a proper MSAA disable
switch.

This fixes graphics corruption in sauerbraten.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=59194