drm/i915: Use a loop for the "three times for luck" DPLL procedure

The magic "enable the  DPLL three times" sequence feels like it
deserves a loop.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170601143619.27840-4-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -1550,6 +1550,7 @@ static void i9xx_enable_pll(struct intel_crtc *crtc)
 	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
 	i915_reg_t reg = DPLL(crtc->pipe);
 	u32 dpll = crtc->config->dpll_hw_state.dpll;
+	int i;
 
 	assert_pipe_disabled(dev_priv, crtc->pipe);
 
@@ -1596,15 +1597,11 @@ static void i9xx_enable_pll(struct intel_crtc *crtc)
 	}
 
 	/* We do this three times for luck */
-	I915_WRITE(reg, dpll);
-	POSTING_READ(reg);
-	udelay(150); /* wait for warmup */
-	I915_WRITE(reg, dpll);
-	POSTING_READ(reg);
-	udelay(150); /* wait for warmup */
-	I915_WRITE(reg, dpll);
-	POSTING_READ(reg);
-	udelay(150); /* wait for warmup */
+	for (i = 0; i < 3; i++) {
+		I915_WRITE(reg, dpll);
+		POSTING_READ(reg);
+		udelay(150); /* wait for warmup */
+	}
 }
 
 /**