drm/i915/fence: replace BUG_ON() with BUILD_BUG_ON()

Avoid BUG_ON(). Since __i915_sw_fence_init() is always called via a
wrapper macro, we can replace it with a compile time BUILD_BUG_ON().

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220830093411.1511040-5-jani.nikula@intel.com