drm/i915: Use I915_READ_FW in i915_get_vblank_counter()

Optimize the multi-register read in i915_get_vblank_counter() a little
bit by grabbing the uncore lock manually and using I915_READ_FW().
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170309154434.29303-2-ville.syrjala@linux.intel.comReviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/i915_irq.c

@@ -723,6 +723,7 @@ static u32 i915_get_vblank_counter(struct drm_device *dev, unsigned int pipe)
 	struct intel_crtc *intel_crtc = intel_get_crtc_for_pipe(dev_priv,
 								pipe);
 	const struct drm_display_mode *mode = &intel_crtc->base.hwmode;
+	unsigned long irqflags;
 
 	htotal = mode->crtc_htotal;
 	hsync_start = mode->crtc_hsync_start;
@@ -739,17 +740,21 @@ static u32 i915_get_vblank_counter(struct drm_device *dev, unsigned int pipe)
 	high_frame = PIPEFRAME(pipe);
 	low_frame = PIPEFRAMEPIXEL(pipe);
 
+	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
+
 	/*
 	 * High & low register fields aren't synchronized, so make sure
 	 * we get a low value that's stable across two reads of the high
 	 * register.
 	 */
 	do {
-		high1 = I915_READ(high_frame) & PIPE_FRAME_HIGH_MASK;
-		low   = I915_READ(low_frame);
-		high2 = I915_READ(high_frame) & PIPE_FRAME_HIGH_MASK;
+		high1 = I915_READ_FW(high_frame) & PIPE_FRAME_HIGH_MASK;
+		low   = I915_READ_FW(low_frame);
+		high2 = I915_READ_FW(high_frame) & PIPE_FRAME_HIGH_MASK;
 	} while (high1 != high2);
 
+	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
+
 	high1 >>= PIPE_FRAME_HIGH_SHIFT;
 	pixel = low & PIPE_PIXEL_MASK;
 	low >>= PIPE_FRAME_LOW_SHIFT;