drm/i915: Add an encoder .shutdown() hook

drm/i915: Add an encoder .shutdown() hook

Add a new encoder hook .shutdown() which will get called at the end
of the pci .shutdown() hook. We shall use this to deal with the
panel power cycle delay issues.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201001151640.14590-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>