[SRU] UBUNTU: [Packaging]: Stop building nvidia

[Thadeu Lima de Souza Cascardo]

[Impact]
Building these modules wastes build time, and also may potentially lead to
unnecessary updates.

[Fix]
Set do_dkms_nvidia = false on amd64.mk.

[Test case]
Build test the kernel.

[Potential regression]
Missing signed bits for these modules, in case users use them without LRMs.