drm/i915: check the power down well on assert_pipe()

If the power well is disabled, we should not try to read its
registers, otherwise we'll get "unclaimed register" messages.

V2: Don't check whether the power well is enabled or not, just check
whether we asked it to be enabled or not: if we asked to disable the
power well, don't use the registers on it, even if it's still enabled.

V3: Fix bug that breaks all non-Haswell machines.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -1214,9 +1214,15 @@ void assert_pipe(struct drm_i915_private *dev_priv,
 	if (pipe == PIPE_A && dev_priv->quirks & QUIRK_PIPEA_FORCE)
 		state = true;
 
+	if (IS_HASWELL(dev_priv->dev) && cpu_transcoder != TRANSCODER_EDP &&
+	    !(I915_READ(HSW_PWR_WELL_DRIVER) & HSW_PWR_WELL_ENABLE)) {
+		cur_state = false;
+	} else {
 		reg = PIPECONF(cpu_transcoder);
 		val = I915_READ(reg);
 		cur_state = !!(val & PIPECONF_ENABLE);
+	}
+
 	WARN(cur_state != state,
 	     "pipe %c assertion failure (expected %s, current %s)\n",
 	     pipe_name(pipe), state_string(state), state_string(cur_state));