[ARM] pxafb: fix building issue of incorrect reference

Commit "d2a34c1 drivers/video: Move dereference after NULL test" introduced
a build error of "fbi->dev->platform_data->smart_update" being unknown type
to the compiler, fix this by removing the unnecessary test of 'fbi'.

Cc: Julia Lawall <julia@diku.dk>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>