drm/amdgpu: Limit the maximum size of contiguous VRAM that can be encapsulated...

drm/amdgpu: Limit the maximum size of contiguous VRAM that can be encapsulated by an instance of DRM memory node

[Why]
Enable 1:1 mapping between VRAM of a DRM node and a scatterlist node

[How]
Ensure construction of DRM node to not exceed specified limit

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>