drm/i915: assert_spin_locked for pipestat interrupt enable/disable

Just to keep the paranoia equal also sprinkle locking asserts over the
pipestat interrupt enable/disable functions.

Again this results in false positives in the interrupt setup. Add
bogo-locking for these and a big comment explaining why it's there and
that it's indeed unnecessary.

v2: Fix up the spelling fail Paulo spotted in comments.

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