drm/i915/fbc: Make FBC check stolen at use time

As a preparation for Xe change stolen memory initialization check to be
done in use-time instead of during initialization.

In case of xe, stolen memory is initialised much later so it can't be
checked during init. There is no specific reason to check this in init for
i915 either -> perform the check in use-time. This also gives us benefit
fbc_no_reason reporting missing initialization being reason for disabled
fbc.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Jeevan B <jeevan.b@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230614051731.745821-3-jouni.hogander@intel.com