drm/i915: Clarify irq_lock locking, special cases

Grab bag for all the special cases:
- i9xx_check_fifo_underruns is only called from crtc_enable hooks,
  i.e. process context.
- i915_enable_asle_pipestat is only called from interrupt postinstall
  hooks. So again process context.
- gen8_irq_power_well_post_enable is called from the runtime pm code,
  which again means process context.
- The open-coded hpd_irq_setup loop in _thaw is also running in process
  context.

So for all of them the plain _irq variant is sufficient.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index b8bd008..8ce1b13 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -686,11 +686,10 @@
 		intel_modeset_init_hw(dev);
 
 		{
-			unsigned long irqflags;
-			spin_lock_irqsave(&dev_priv->irq_lock, irqflags);
+			spin_lock_irq(&dev_priv->irq_lock);
 			if (dev_priv->display.hpd_irq_setup)
 				dev_priv->display.hpd_irq_setup(dev);
-			spin_unlock_irqrestore(&dev_priv->irq_lock, irqflags);
+			spin_unlock_irq(&dev_priv->irq_lock);
 		}
 
 		intel_dp_mst_resume(dev);