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

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

All of the drivers in the kernel tree only call this from one of the
following contexts:

* drm_crtc_funcs->reset
* During initial module load

Since both of these contexts are guaranteed to have interrupts enabled
beforehand, there's no need to use the irqsave/irqrestore variants of
spin_(un)lock(). So, fix this 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/20200720190736.180297-2-lyude@redhat.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>