Ensure waiting loops terminate during cold boot. This fixes boot with
MacBookPro and i915kms_load="YES" in /boot/loader.conf.

A lowlevel timeout in one of the display ports caused an infinite wait
because a ticks/jiffies comparison was constant. The clock subsystem
which makes ticks/jiffies increment is started after the initial
driver probing is done. Refer to sys/kernel.h and SI_SUB_DRIVERS vs
SI_SUB_CLOCKS .

Discussed with: kmacy @