mesa: Add driver method to determine the possible sample counts

mesa: Add driver method to determine the possible sample counts

Use this method in _mesa_GetInternalformativ for both GL_SAMPLES and
GL_NUM_SAMPLE_COUNTS.

v2: internalFormat may not be color renderable by the driver, so zero
can be returned as a sample count.  Require that drivers supporting the
extension provide a QuerySamplesForFormat function.  The later was
suggested by Eric Anholt.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>