st/mesa: only try to create 1x msaa surfaces for "fake" msaa drivers

st/mesa: only try to create 1x msaa surfaces for "fake" msaa drivers

For software drivers where we want "fake" msaa support for GL 3.x, we
treat 1 sample as being msaa.

For drivers with real msaa support, start format probing at 2x msaa.
For drivers with fake msaa support, start format probing at 1x msaa.

This also tweaks the MaxSamples code in st_init_extensions() so that
we use MaxSamples=1 for fake msaa.  This allows the format proble loops
to run at least one iteration.

This fixes a llvmpipe/VTK regression from commit 6839d3369905eb02151.
And for drivers with fake msaa support, calls such as
glTexImage2DMultisample(samples=1) will now succeed.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102038
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102125
Reviewed-by: Roland Scheidegger <sroland@vmware.com>