drm/i915: rework dev->first_error locking

- reduce the irq disabled section, even for a debugfs file this was
  way too long.
- always disable irqs when taking the lock.

v2: Thou shalt not mistake locking for reference counting, so:
- reference count the error_state to protect from concurent freeeing.
  This will be only really used in the next patch.

Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>