At the moment we have the following interrupt enabling sequence:
1. irq preinstall hook
2. enabling the interrupt handler and calling irq postinstall hook
3. enable rps interrupts from the async work

And the folliwing disable sequence:
1. disabling the interrupt handler and calling the uninstall hook
2. disabling the rps interrupt

Since the postinstall hook now always sets up PMIIR, PMIER and PMIMR
to known-good states there no way for an interrupt to sneak in in the
enable sequence, so we can reinstate the WARN lost in

commit eda63ffb
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Tue May 28 19:22:26 2013 -0700

    drm/i915: Add PM regs to pre/post install

Note that there's some room for future cleanups since most of the
interrupt register clearing in the disable function is rather
redundant. But that's better done in follow-up patches, if at all.

Cc: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>