drm/i915/ddi: only call shutdown hooks for valid encoders

drm/i915/ddi: only call shutdown hooks for valid encoders

DDI might be HDMI or DP only, leaving the other encoder
uninitialized. Calling the shutdown hook on an uninitialized encoder may
lead to a NULL pointer dereference. Check the encoder types (and thus
validity via the DP output_reg or HDMI hdmi_reg checks) before calling
the hooks.

Reported-and-tested-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Closes: https://lore.kernel.org/r/20241031105145.2140590-1-senozhatsky@chromium.org
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8b197c50e7f3be2bbc07e3935b21e919815015d5.1735568047.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>