iris: Zero the add-on clear color BO on import

iris: Zero the add-on clear color BO on import

When iris imports an I915_FORMAT_MOD_Y_TILED_GEN12_RC_CCS surface, it
allocates a buffer to hold the indirect clear color. When the import is
complete, iris_resource::aux::clear_color is set to zero but the
indirect buffer is filled with garbage values. This could break certain
texture view use-cases, so zero the allocated buffer to fix those.

Fixes: c19492bcdb9 ("iris: Handle importing aux-enabled surfaces on TGL")
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6092>