st/mesa: don't fall back to 16F when 32F is requested

st/mesa: don't fall back to 16F when 32F is requested

Nothing in the spec allows for the reduced precision, and this also
fixes st_QuerySamplesForFormat for nv50, which does not allow MS8 on
RGBA32F. Now this will be respected instead of reporting MS8 as
supported with an assumption that the format used will be RGBA16F.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>