linuxkpi: Add pin_user_pages*() functions

They were defined in the i915 DRM driver. I move the code in linuxkpi so
it can benefit other drivers.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38078

(cherry picked from commit 3d751b7a719fd7934e902a34285ce4e5bbaf1e16)