Staging: xgifb: Remove unused spinlock in struct video_info

xgifb framebuffer driver had an option to use an accelerator engine that never
got used (XGIfb_accel was always 0).

An earlier patchset removed the code relevant to the accelerator.
Since this spinlock was used only for that code, it can be deleted as well.

Signed-off-by: Javier Martinez Canillas <martinez.javier@gmail.com>
Acked-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>