LinnuxKPI: drm2: retire timespec_to_jiffies()

[bz]

Summary

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

Test Plan

grep for the function name in sys, and
drm-kmod 5.4-lts, 5.10-lts, 5.15-lts, and 6.1-lts.