drm/i915/fbc: replace GEM_BUG_ON() to drm_WARN_ON()

Avoid using GEM_BUG_ON() in display code.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a7d53a403822b43c7d78689a10480b47ccc0534d.1694684044.git.jani.nikula@intel.com