i965/fs: Use exec_size instead of dst.width for computing component size

i965/fs: Use exec_size instead of dst.width for computing component size

There are a variety of places where we use dst.width / 8 to compute the
size of a single logical channel.  Instead, we should be using exec_size.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Acked-by: Francisco Jerez <currojerez@riseup.net>