drm: vblank cannot be enabled if dev->irq_enabled is false

drm: vblank cannot be enabled if dev->irq_enabled is false

Since we cannot enable the vblank if !dev->irq_enabled, we assert that
checking for both !vblank->enabled and !dev->irq_enabled is tautological
and only need the former. The only time it may differ is when racing
with drm_irq_uninstall(), but that will then disable the vblank and
wakeup the waiters.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170317202030.24410-2-chris@chris-wilson.co.uk
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>