minor improvements to the Makefile target "dkms-install"

[feiyax]

I just attempted to install it after a fresh minimal install of ubuntu 20.04, then of course it failed due to no dkms. After installing dkms via package manager, it failed again due to mkdir'ing an existing path, which was the residue from the last unsuccessful attempt.

Suggesting to make changes so that:

1. add -p arg to mkdir so that it doesn't fail if the path exists already
2. test the dkms availability before any action in the dkms-install target, so that it will neither create directory nor copy those paths if dkms is unavailable.