drm/amdgpu: Fix size validation for non-exclusive domains (v4)

Fix amdgpu_bo_validate_size() to check whether the TTM domain manager for the
requested memory exists, else we get a kernel oops when dereferencing "man".

v2: Make the patch standalone, i.e. not dependent on local patches.
v3: Preserve old behaviour and just check that the manager pointer is not
    NULL.
v4: Complain if GTT domain requested and it is uninitialized--most likely a
    bug.

Cc: Alex Deucher <Alexander.Deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: AMD Graphics <amd-gfx@lists.freedesktop.org>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>