drm/i915: skip redundant operations whilst enabling pipes and planes

If the pipe or plane is already enabled, then we do not need to enable
it again and can skip the delay. Similarly if it is already disabled
when we want to disable it, we can also skip it.

This fixes a regression from b24e717988, which caused the LVDS
output on one PineView machine to become corrupt after changing
orientation several times.

References: https://bugs.freedesktop.org/show_bug.cgi?id=34601
Cc: Jesse Barnes <[e-mail]>
Signed-off-by: Chris Wilson <[e-mail]>
Reviewed-by: Keith Packard <[e-mail]>
Tested-by: [e-mail]

drivers/gpu/drm/i915/intel_display.c

···
 
 	reg = PIPECONF(pipe);
 	val = I915_READ(reg);
-	val |= PIPECONF_ENABLE;
-	I915_WRITE(reg, val);
+	if (val & PIPECONF_ENABLE)
+		return;
+
+	I915_WRITE(reg, val | PIPECONF_ENABLE);
 	intel_wait_for_vblank(dev_priv->dev, pipe);
 }
 
···
 
 	reg = PIPECONF(pipe);
 	val = I915_READ(reg);
-	val &= ~PIPECONF_ENABLE;
-	I915_WRITE(reg, val);
+	if ((val & PIPECONF_ENABLE) == 0)
+		return;
+
+	I915_WRITE(reg, val & ~PIPECONF_ENABLE);
 	intel_wait_for_pipe_off(dev_priv->dev, pipe);
 }
 
···
 
 	reg = DSPCNTR(plane);
 	val = I915_READ(reg);
-	val |= DISPLAY_PLANE_ENABLE;
-	I915_WRITE(reg, val);
+	if (val & DISPLAY_PLANE_ENABLE)
+		return;
+
+	I915_WRITE(reg, val | DISPLAY_PLANE_ENABLE);
 	intel_wait_for_vblank(dev_priv->dev, pipe);
 }
 
···
 
 	reg = DSPCNTR(plane);
 	val = I915_READ(reg);
-	val &= ~DISPLAY_PLANE_ENABLE;
-	I915_WRITE(reg, val);
+	if ((val & DISPLAY_PLANE_ENABLE) == 0)
+		return;
+
+	I915_WRITE(reg, val & ~DISPLAY_PLANE_ENABLE);
 	intel_flush_display_plane(dev_priv, plane);
 	intel_wait_for_vblank(dev_priv->dev, pipe);
 }