[intel] Use IMR instead of IER to pend interrupts during ISR

Noting that the interrupt mask register was more reliable than the interrupt
enable register for managing interrupts in user_irq_on/user_irq_off, this
patch replaces the remaining IER frobbing with IMR instead.

The test which exposes IER related failures is:

$ glxgears & glxgears & glxgears
(reposition the glxgears windows away from the upper left corner)
$ while :; do x11perf -rect100 -reps 800 -repeat 1; sleep 1; done &
$ while :; do runoa; runet; done &