panfrost: Use u_default_clear_buffer

[Alyssa: This is required for OpenCL.]

Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <!16044>
164 jobs for !16044 with gallium/u-clear-buffer in 37 minutes and 34 seconds (queued for 10 seconds)
latest merge request