i965/msaa: Control multisampling behaviour via the visual.

i965/msaa: Control multisampling behaviour via the visual.

Previously, we used the number of samples in draw buffer 0 to
determine whether to set up the 3D pipeline for multisampling.  Using
the visual is cleaner, and has the benefit of working properly when
there is no color buffer.

Fixes all piglit tests "EXT_framebuffer_multisample/no-color" on Gen7.
On Gen6, the "depth-computed" variants of these tests still fail; this
will be addresed in a later patch.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>