drm/i915: synchronize_irq() against the actual irq

drm/i915: synchronize_irq() against the actual irq

When eliminating our use of drm_irq_install() I failed to convert
all our synchronize_irq() calls to consult pdev->irq instead of
dev_priv->drm.irq. As we no longer populate dev_priv->drm.irq
we're no longer synchronizing against anything.

v2: Add intel_syncrhonize_irq() (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reported-by: Imre Deak <imre.deak@intel.com>
Fixes: 92893a8bfadf ("drm/i915: Nuke drm_driver irq vfuncs")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111012
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190702151723.29739-1-ville.syrjala@linux.intel.com