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

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

This is only called from:
* Atomic modesetting hooks
* Module probing routines
* Legacy modesetting hooks

All of which have IRQs enabled, so we can also get rid of
irqsave/restore here to make the IRQ context of this function more
obvious.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200720190736.180297-3-lyude@redhat.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>