drm/i915: Check mask/bit helper functions

After a bit of irc discussion we've concluded that it would be prudent
to check that callers use the mask/enable paramters correctly. So add
a WARN_ON.

Spurred by Damien's bugfix which added _MASKED_FIELD.

v2: We use WARN_ON(1) a lot to catch default cases in switch blocks
which should always be extended. So this doesn't work really. Dunno
why gcc only started complaining when I've moved the WARN out of the
static inline helper to address a feedback from Jani.

Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/i915/i915_irq.c

@@ -183,6 +183,8 @@ static void ilk_update_gt_irq(struct drm_i915_private *dev_priv,
 {
 	assert_spin_locked(&dev_priv->irq_lock);
 
+	WARN_ON(enabled_irq_mask & ~interrupt_mask);
+
 	if (WARN_ON(!intel_irqs_enabled(dev_priv)))
 		return;
 
@@ -229,6 +231,8 @@ static void snb_update_pm_irq(struct drm_i915_private *dev_priv,
 {
 	uint32_t new_val;
 
+	WARN_ON(enabled_irq_mask & ~interrupt_mask);
+
 	assert_spin_locked(&dev_priv->irq_lock);
 
 	new_val = dev_priv->pm_irq_mask;
@@ -328,6 +332,8 @@ void ibx_display_interrupt_update(struct drm_i915_private *dev_priv,
 	sdeimr &= ~interrupt_mask;
 	sdeimr |= (~enabled_irq_mask & interrupt_mask);
 
+	WARN_ON(enabled_irq_mask & ~interrupt_mask);
+
 	assert_spin_locked(&dev_priv->irq_lock);
 
 	if (WARN_ON(!intel_irqs_enabled(dev_priv)))