drm/i915: vlv: fix display IRQ enable/disable

drm/i915: vlv: fix display IRQ enable/disable

We want to enable/disable display IRQs only if global i915 IRQs are
enabled. To check the latter it's not enough to consult the DRM
dev->irq_enabled flag, since runtime PM can disable/enable IRQs
and it won't adjust this flag only the i915 specific
dev_priv->pm._irqs_disabled flag. Fix this by using the proper
intel_irqs_enabled() helper instead.

Fortunately this didn't cause an actual problem since even if we enabled
display IRQs too early (before enabling global i915 IRQs) the
VLV_MASTER_IER would still be clear masking all IRQs.

This issue was caught by

commit 920dd15a2b2fc60d054646a8a1ffd6aeb6090e05
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Aug 27 10:43:37 2014 +0200

    drm/i915: WARN if interrupts aren't on in en/disable_pipestat

Signed-off-by: Imre Deak <imre.deak@intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>