Work functions are in process context, so plain _irq spinlock variants
is all we need.

The hpd reenable work didn't follow the _work/_work_func postfix
naming scheme, so adjust that while at it.
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>