mesa: only check sample count if we actually wanted multisampling

mesa: only check sample count if we actually wanted multisampling

Fixes various test fallout from 90b5a2425a on Pineview, which claims to
support ARB_internalformat_query but doesn't actually provide the
driverfunc.

That driver is still broken [GetInternalformativ will still segfault!]
but it was silly to be going through the sample count logic in the
nonmultisampling case at all.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>