drm/amdgpu/mes: fix vm csa update issue

Need reserve VM buffers before update VM csa.

v2: rebase fixes

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>