drm/i915: check the power well when redisabling VGA

If the power well is disabled VGA is guaranteed to be disabled.

This fixes unclaimed register messages that happen on suspend/resume.

v2: Check the actual hw power well state instead of our own tracking
to make sure VGA is _really_ off (in case the BIOS/KVMr has just its
own request bit set). Requested by Ville.

Note: Ville suggested whether it wouldn't be better to just enable the
power well over a slightly longer time in our resume code, since we
already do that. I tend to agree, but there's also the modeset force
code in the lid notifier which _also_ eventually calls redisable_vga.
We shouldn't ever need this on somewhat modern hw (everything with
opregion essentially) but the code to bail out isn't there. Hence
stick with this simple approach here for now.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67517
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Summarize the discussion around the resume sequence and lid
notifier a bit.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -10104,6 +10104,17 @@ void i915_redisable_vga(struct drm_device *dev)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	u32 vga_reg = i915_vgacntrl_reg(dev);
 
+	/* This function can be called both from intel_modeset_setup_hw_state or
+	 * at a very early point in our resume sequence, where the power well
+	 * structures are not yet restored. Since this function is at a very
+	 * paranoid "someone might have enabled VGA while we were not looking"
+	 * level, just check if the power well is enabled instead of trying to
+	 * follow the "don't touch the power well if we don't need it" policy
+	 * the rest of the driver uses. */
+	if (HAS_POWER_WELL(dev) &&
+	    (I915_READ(HSW_PWR_WELL_DRIVER) & HSW_PWR_WELL_STATE) == 0)
+		return;
+
 	if (I915_READ(vga_reg) != VGA_DISP_DISABLE) {
 		DRM_DEBUG_KMS("Something enabled VGA plane, disabling it\n");
 		i915_disable_vga(dev);