drm/i915: Do not set any power wells when there is no display

Power wells are only part of display block and not necessary when
running a headless driver.

Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210408203150.237947-2-jose.souza@intel.com