drm/i915: Fix deadlock in i830_disable_pipe()

i830_disable_pipe() gets called from the power well code, and thus
we're already holding the power domain mutex. That means we can't
call plane->get_hw_state() as it will also try to grab the
same mutex and will thus deadlock.

Replace the assert_plane() calls (which calls ->get_hw_state()) with
just raw register reads in i830_disable_pipe(). As a bonus we can
now get a warning if plane C is enabled even though we don't even
expose it as a drm plane.

v2: Do a separate WARN_ON() for each plane (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 51f5a0963984 ("drm/i915: Add .get_hw_state() method for planes")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171129125411.29055-1-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/intel_display.c

@@ -14770,8 +14770,11 @@ void i830_disable_pipe(struct drm_i915_private *dev_priv, enum pipe pipe)
 	DRM_DEBUG_KMS("disabling pipe %c due to force quirk\n",
 		      pipe_name(pipe));
 
-	assert_planes_disabled(intel_get_crtc_for_pipe(dev_priv, PIPE_A));
-	assert_planes_disabled(intel_get_crtc_for_pipe(dev_priv, PIPE_B));
+	WARN_ON(I915_READ(DSPCNTR(PLANE_A)) & DISPLAY_PLANE_ENABLE);
+	WARN_ON(I915_READ(DSPCNTR(PLANE_B)) & DISPLAY_PLANE_ENABLE);
+	WARN_ON(I915_READ(DSPCNTR(PLANE_C)) & DISPLAY_PLANE_ENABLE);
+	WARN_ON(I915_READ(CURCNTR(PIPE_A)) & CURSOR_MODE);
+	WARN_ON(I915_READ(CURCNTR(PIPE_B)) & CURSOR_MODE);
 
 	I915_WRITE(PIPECONF(pipe), 0);
 	POSTING_READ(PIPECONF(pipe));