drm/i915: fix HPD IRQ reenable work cancelation

Atm, the HPD IRQ reenable timer can get rearmed right after it's
canceled. Also to access the HPD IRQ mask registers we need to wake up
the HW.

Solve both issues by converting the reenable timer to a delayed work and
grabbing a runtime PM reference in the work. By this we can also forgo
canceling the timer during runtime suspend, since the only important
thing there is that the HW is awake when we write the registers and
that's ensured by the RPM ref. So do the cancelation only during driver
unload time; this is also a requirement for an upcoming patch where we
want to cancel all HPD related works only during system suspend and
driver unload time, but not during runtime suspend.

Note that there is still a race between the HPD IRQ reenable work and
drm_irq_uninstall() during driver unload, where the work can reenable
the HPD IRQs disabled by drm_irq_uninstall(). This isn't a problem since
the HPD IRQs will still be effectively masked by the first level
interrupt mask.

v2-3:
- unchanged
v4:
- use proper API for changing the expiration time for an already pending
  delayed work (Jani)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v2)
Cc: stable@vger.kernel.org (3.16+)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>