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

Nanley Chery requested to merge nchery/mesa:zero-add-on-cc into master

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: c19492bc ("iris: Handle importing aux-enabled surfaces on TGL")

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