drm/xe/debugfs: Add node to dump guc log to dmesg

drm/xe/debugfs: Add node to dump guc log to dmesg

Currently xe_guc_log_print_dmesg() is unused, as it's expected
developers to add those calls when needed. However it makes it hard to
guarantee it's working as nothing is testing it. Add a node in debugfs
so it can be tested. This is purely for testing purposes since with the
device probed and working, the guc log can be obtained by the regular
debugfs file.

Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250131171716.3998432-1-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>