drm/i915: Mark i915_hpd_poll_init_work as static

drm/i915: Mark i915_hpd_poll_init_work as static

Local function with forgotten static declaration.

Fixes: 19625e85c6ec ("drm/i915: Enable polling when we don't have hpd")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Lyude <cpaul@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471432146-5196-2-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>