drm/amd/ras: make UNIRAS CPER debugfs header legacy-compatible

drm/amd/ras: make UNIRAS CPER debugfs header legacy-compatible

The UNIRAS CPER debugfs path returned a zeroed 12-byte prefix and used
file offset directly as the CPER record index. Legacy CPER ring readers
expect the prefix to contain three 32-bit ring pointers followed
immediately by CPER payload data.

Build the same header shape for UNIRAS reads by reporting a zero read
pointer and matching write pointers for the returned payload size. Keep
an internal record cursor behind the debugfs offset so follow-up reads
continue from the correct CPER record while first reads still expose the
legacy prefix.

Signed-off-by: Xiang Liu <xiang.liu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>