drm/i915: add helper for checking whether IRQs are enabled

Now that we use the runtime IRQ enable/disable functions in our suspend
path, we can simply check the pm._irqs_disabled flag everywhere.  So
rename it to catch the users, and add an inline for it to make the
checks clear everywhere.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>