drm/i915: Track IRQ state in local device state

Replace usage of struct drm_device.irq_enabled with the driver's
own state field struct drm_i915_private.irq_enabled. The field in
the DRM device structure is considered legacy and should not be
used by KMS drivers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210625082222.3845-7-tzimmermann@suse.de