radeonsi: fix Polaris MSAA regression

radeonsi: fix Polaris MSAA regression

The regression was introduced by commit d938b8c. The problem here is that in
order to use the small primitive filter, we need to explicitly set the sample
locations to 0. But the DB doesn't properly process the change of sample
locations without a flush, and so we can end up with incorrect Z values.

Instead of doing a flush, just disable the small primitive filter when MSAA
is force-disabled.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96908
Cc: 12.0 <mesa-stable@lists.freedesktop.org>