drm/amdgpu: Only create mes event log debugfs when mes is enabled

Skip the debugfs file creation for mes event log if the GPU
doesn't use MES. This to prevent potential kernel oops when
user try to read the event log in debugfs on a GPU without MES

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