Allow controlling pr_debug at runtime in the LinuxKPI.

Turning on pr_debug at compile time make it non-optional at runtime.
This often means that the amount of the debugging is unbearable.
Allow developer to turn on pr_debug output only when needed.

Build tested drm-current-kmod prior to commit.

MFC after: 1 week
Submitted by: kib@
Sponsored by: Mellanox Technologies