drm/i915: Introduce intel_hpd_hotplug_irqs()

Introduce intel_hpd_hotplug_irqs() as a partner to
intel_hpd_enabled_irqs(). There's no need to care about the
encoders which we're not exposing, so we can avoid hardcoding
the masks in various places.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200630215601.28557-12-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>