drm/i915/display: Move enable_sagv module parameter under display

Move enable_sagv module parameter under display and change the parameter
permissions to non-writblase (0400)

v2: Change permissions to 0400

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231024124109.384973-12-jouni.hogander@intel.com