anv: Initalize the clear color struct for CNL+
On CNL+, the clear color struct is composed of RGBA channel values and fields which are either reserved by the HW or used to control fast-clears. Currently anv initializes the channel values to zero and allows the other fields to be undefined.
Satisfy the MBZ field requirements by removing an optimization that doesn't hold true for CNL+ and pulling in the number of dwords to initialize from ISL.