video: fbdev: sm501fb: fix potential null pointer dereference on fbi

The pointer fbi is dereferenced with par = fbi->par before there is a
null check on fbi, hence there is a potential null pointer dereference
on a null par.  Fix this by moving the dereference after the null
pointer check.

Detected by CoverityScan, CID#1461301 ("Dereference before null check")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>