intel/pre-gen6: Disable EXT_framebuffer_multisample.

Previously, the i965 driver enabled EXT_framebuffer_multisample even
on pre-gen6 chipsets.  However, since we don't support multisampling
on these chips, we set GL_MAX_SAMPLES=1 (the minimum allowed by
EXT_framebuffer_multisample), and if the client ever requested a
multisample buffer, we quietly supplied them with a single-sampled
buffer instead.

After some discussion on the mailing list (see thread
"ext_framebuffer_multisample: check for num_samples<=1"), it's clear
that this was the wrong approach.  The correct approach is to only
expose EXT_framebuffer_multisample when we truly support
multisampling; that frees us to set a sensible value of
GL_MAX_SAMPLES=0 on other chipsets, so that we never have to deal with
a client requesting a multisample buffer when multisampling isn't
supported.

This change causes the following piglit tests to be skipped on
chipsets prior to Gen6:

- "ARB_framebuffer_sRGB/blit {renderbuffer,texture}
  {linear,linear_to_srgb,srgb,srgb_to_linear}
  {downsample,msaa,upsample} {disabled,enabled}"
- EXT_framebuffer_multisample/blit-mismatched-formats
- EXT_framebuffer_multisample/blit-mismatched-sizes
- EXT_framebuffer_multisample/dlist
- EXT_framebuffer_multisample/interpolation 0 *
- EXT_framebuffer_multisample/minmax
- EXT_framebuffer_multisample/negative-copypixels
- EXT_framebuffer_multisample/negative-copyteximage
- EXT_framebuffer_multisample/negative-max-samples
- EXT_framebuffer_multisample/negative-mismatched-samples
- EXT_framebuffer_multisample/negative-readpixels
- EXT_framebuffer_multisample/renderbuffer-samples
- EXT_framebuffer_multisample/renderbufferstorage-samples
- EXT_framebuffer_multisample/samples

This is expected, since the above tests exercise MSAA functionality,
and shouldn't be run on systems prior to Gen6.