drm/i915: Use debugfs_create_bool() for "i915_bigjoiner_force_enable"

There is no reason to make this debugfs file for a simple
boolean so complicated. Just use debugfs_create_bool().

Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240402135148.23011-8-ville.syrjala@linux.intel.com