mesa/main: consider multisampling enabled when number of samples == 1

mesa/main: consider multisampling enabled when number of samples == 1

There are some differences between how non-multisampled framebuffers (i.e.
samples == 0) and multisampled framebuffers with a single sample should be
treated.  For example, alpha to coverage and writing to gl_SampleMask has an
effect with single-sample multisample framebuffers, but not on
non-multisample framebuffers.

This fixes GL45-CTS.sample_variables.mask.*.samples_1.* at least for
Gallium drivers (and possibly others, though at least radeonsi needs an
additional fix).

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>