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

This function is only ever called from ioctl context, so we're
guaranteed to have interrupts enabled. Stop using the irqsave/irqrestore
variants of spin_(un)lock_irq() to make this more obvious.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200720190736.180297-4-lyude@redhat.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>