sm501fb: deallocate colormap only if allocated

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 deallocate the colormap only if that particular fb is defined.

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