drm/amdgpu: cleanup visible vram size handling

Centralize the limit handling and validation in one place instead
of spreading that around in different hw generations.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>