drm/amdgpu: SW part of MES event log enablement

This is the generic SW part, prepare the event log buffer and dump it through debugfs

Signed-off-by: shaoyunl <shaoyun.liu@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>