gallium: Fix util_format_get_depth_only

This is a pipe format, not a boolean.

Fixes: 5849e061 ("gallium/auxiliary: Add util_format_get_depth_only() helper.")
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
22 jobs for !1910 with pipe-format-fix in 3 minutes and 38 seconds (queued for 1 minute and 21 seconds)
merge request