drm/i915: Warn on if set frontbuffer return value is not NULL on release

i915_gem_object_set_frontbuffer returns set frontbuffer pointer.  When we
are releasing frontbuffer we are clearing the pointer from the object. Warn
on if return value is not null.

v3: Check i915_gem_object_set_frontbuffer return value separately
v2: Instead of ignoring do drm_WARN_ON

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230928082924.164720-1-jouni.hogander@intel.com