drm/i915: Remove I915_HAS_HOTPLUG() check from i915_hpd_irq_setup()

The dev_priv->display.hpd_irq_setup hook is optional, so we can move the
I915_HAS_HOTPLUG() check out of i915_hpd_irq_setup() and only set up the
hook when hotplug support is present.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>