sm501fb: unregister framebuffer only if registered

There are cases when panel and crt both are not defined and only one of
them is defined and initialized. In such cases, while removing the
device, unregister the framebuffer only if it was registered.

Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>