drivers/video/xen-fbfront.c: add missing cleanup code

The operations in the subsequent error-handling code appear to be also
useful here.

Acked-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
[v1: Collapse some of the error handling functions]
[v2: Fix compile warning]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>