drm/amdgpu: add the per-context meta data v3

The per-context meta data is a per-context data structure associated
with a mes-managed hardware ring, which includes MCBP CSA, ring buffer
and etc.

v2: fix typo
v3: a. use structure instead of typedef
    b. move amdgpu_mes_ctx_get_offs_* to amdgpu_ring.h
    c. use __aligned to make alignement

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