drm/i915: Remove NULL dev_priv checks from irq_uninstall

There should be no way to land in irq_uninstall without a
valid dev_priv. Let's kill off the remaining checks, which are
probably some kind of UMS leftovers. Not all the irq_uninstall
hooks even had them anymore.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818183705.27850-10-ville.syrjala@linux.intel.com