anv/cmd_buffer: 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.

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
(cherry picked from commit b4198e79)