i965: Move fs_inst::has_side_effects()'s eot check to the parent class.

i965: Move fs_inst::has_side_effects()'s eot check to the parent class.

This eliminates a layer of wrapping, and makes a backend_instruction
sufficient.  The downside is that it exposes 'eot' to the vec4 backend,
which it doesn't need, but can basically happily ignore.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Tested-by: Pallavi G <pallavi.g@intel.com>