drm/vblank: Use spin_(un)lock_irq() in drm_queue_vblank_event()

This one's easy - we're already calling kzalloc(GFP_KERNEL) in this
function, so we must already be guaranteed to have IRQs enabled when
calling this. So, use the plain _irq() variants of spin_(un)lock() to
make this more obvious.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200720190736.180297-5-lyude@redhat.com