drm/i915: Guard debugfs against invalid access without display

Do not create the display debugfs files when we don't have display.

Based on previous patch by José Souza.

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201027044618.719064-1-lucas.demarchi@intel.com