mesa/virgl: Fix off-by-one and copy-paste error in multisample position evaluation

mesa/virgl: Fix off-by-one and copy-paste error in multisample position evaluation

Converting from a switch statement that would not allow intermediate sample counts
to use an if-else chain went a bit wrong, so that in some cases the range that
should be inclusive was exclusive and the line for 16 samples was copies wrongly.

v2: elaborate commit message.

Fixes: 91f48cdfe5c817158c533a8f67c60e9aabbe4479
       virgl: Add support for glGetMultisample
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com> (v1)