drm/amdgpu: Make contiguous pinning optional

Enable pinning of VRAM without forcing it to be contiguous. When memory is
already pinned, make sure it's contiguous if requested.

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