i965/fs: Force per-sample dispatch if the shader reads from a multisample FBO.

i965/fs: Force per-sample dispatch if the shader reads from a multisample FBO.

The result of a framebuffer fetch from a multisample FBO is inherently
per-sample, so the spec requires at least those sections of the shader
that depend on the framebuffer fetch result to be executed once per
sample.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>