intel: Advertise multisample DRI2 configs on gen >= 6

This turns on window system MSAA.

This patch changes the id of many GLX visuals and configs, but that
couldn't be prevented. I attempted to preserve the id's of extant configs
by appending the multisample configs to the end of the extant ones. But
somewhere, perhaps in the X server, the configs are reordered with
multisample configs interspersed among the singlesample ones.

Test results:
  Tested with xonotic and `glxgears -samples 1` on Ivybridge.

  No piglit regressions on Ivybridge.

  On Sandybridge, passes 68/70 of oglconform's
  winsys multisample tests.  The two failing tests are:
      multisample(advanced.pixelmap.depth)
      multisample(advanced.pixelmap.depthCopyPixels)
  These tests hang the gpu (on kernel 3.4.6) due to
  a glDrawPixels/glReadPixels pair on an MSAA depth buffer.  I don't expect
  realworld apps to do that, so I'm not too concerned about the hang.

  On Ivybridge, passes 69/70. The failing case is
  multisample(advanced.line.changeWidth).

Reviewed-by: Eric Anholt <[email protected]>
Signed-off-by: Chad Versace <[email protected]>

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