drm/i915: Convert intel_hpd_irq_event() into an encoder hotplug hook

Allow encoders to customize their hotplug processing by moving the
intel_hpd_irq_event() code into an encoder hotplug vfunc. Currently
only SDVO needs this to re-enable hotplug signalling in the SDVO
chip. We'll use this same hook for DP/HDMI link management later.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180117192149.17760-1-ville.syrjala@linux.intel.com