drm/amdgpu/mes: set correct mes ring ready flag

Set corresponding ready flag for mes ring when enable or disable
mes ring.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>