ps3fb: prevent use after free of fb_info

In ps3fb_shutdown, freeing the framebuffer will cause fb_info (in
dev->core.driver_data) to be free()ed, which we potentially access
from the ps3fbd kthread.

This change frees the framebuffer after stopping the ps3fbd kthread.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>