LinnuxKPI: drm2: retire timespec_to_jiffies()

Linux has removed timespec_to_jiffies() half a decade ago [1].
I cannot find any use of it anymore in recent drm-kmod branches
or in the tree so retire it.

While here also retire it from drm2.

Reported by: emaste (D48318) [1].
Sponsored by: The freeBSD Foundation
MFC after: 2 weeks
Reviewed by: emaste, dumbbell (tested all drm-kmod versions, thanks!)
Differential Revision: https://reviews.freebsd.org/D48379