i965/fs: Add a sampler program key for whether the texture is 16x MSAA

i965/fs: Add a sampler program key for whether the texture is 16x MSAA

When 16x MSAA is used for sampling with texelFetch the compiler needs
to use a different instruction which passes more arguments for the MCS
data. Previously on skl+ it was unconditionally using this new
instruction. However since 16x MSAA is probably going to be pretty
rare, it is probably worthwhile to avoid using this instruction for
the other sample counts. In order to do that this patch adds a new
member to brw_sampler_prog_key_data to track when a sampler refers to
a buffer with 16 samples.

Note that this isn't done for the vec4 backend because it wouldn't
change how many registers it uses.

Acked-by: Ben Widawsky <ben@bwidawsk.net>