drm/i915: Clarify irq_lock locking, intel_tv_detect

->detect callbacks are only ever called from process context, and
there's no fancy nesting going on here. So plain _irq spinlock
variants is what we want.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>