isl_dev.ss.clear_color_state_size is already zero on BDW and SKL. Drop
the redundant platform check and return the field directly.
We're going to have this function return zero more often and it will do
so uniformly using if-statements. We choose to remove the redundant
expression instead of adding a redundant if-statement.
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13555>
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