LinuxKPI: Finalize move of lindebugfs from ports to base.

The source file was moved to base earlier and also improved upon,
but never compiled in. This patch will:

• Make a module in sys/modules
• Make lindebugfs depend on linuxkpi (for seq_file)
• Check if read/write functions are set before calling, DRM drivers don't always set both of them.

Reviewed by: hps
Approved by: imp (mentor), hps
MFC after: 1 week