sm501fb: suspend and resume fb if it exists

There are cases when panel and crt both are not defined and only one of
them is defined and initialized. In such cases, suspend or resume it
only if it is defined.

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