i965/msaa: Only do multisample rasterization if GL_MULTISAMPLE enabled.

i965/msaa: Only do multisample rasterization if GL_MULTISAMPLE enabled.

From the GL 3.0 spec (p.116):

    "Multisample rasterization is enabled or disabled by calling
    Enable or Disable with the symbolic constant MULTISAMPLE."

Elsewhere in the spec, where multisample rasterization is described
(sections 3.4.3, 3.5.4, and 3.6.6), the following text is consistently
used:

    "If MULTISAMPLE is enabled, and the value of SAMPLE_BUFFERS is
    one, then..."

So, in other words, disabling GL_MULTISAMPLE should prevent
multisample rasterization from occurring, even if the draw framebuffer
is multisampled.  This patch implements that behaviour by setting the
WM and SF stage's "multisample rasterization mode" to
MSRAST_ON_PATTERN only when the draw framebuffer is multisampled *and*
GL_MULTISAMPLE is enabled.

Fixes piglit test spec/EXT_framebuffer_multisample/enable-flag.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>