Fix segfault when both fb_copyarea and hardware cursor are enabled

This patch prevents a segmentation fault caused by a logic issue in file **src/fbdev.c**:

https://github.com/lgeek/xf86-video-fbturbo/blob/master/src/fbdev.c#L929
If `fPtr->fb_copyarea_private` is NOT NULL, then `fPtr->RkFb_private` is not initialized.

https://github.com/lgeek/xf86-video-fbturbo/blob/master/src/fbdev.c#L1118
Then `RkDispHardwareCursor_Init()` crashes when trying to access `rkfb->fb_fd`.