i965/fs: Use exec_size instead of dispatch_width to determine the message variant.

i965/fs: Use exec_size instead of dispatch_width to determine the message variant.

dispatch_width is global for a single compilation and doesn't
necessarily match the desired execution width if we had to lower the
original full-width instruction due to hardware limitations.  These
were all inside a Gen4-specific branch so this patch shouldn't have
any effect on more recent hardware.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>