drm/gma500: remove unnecessary stub for fb_ioctl()

Stub implementation of fb_ioctl can be omitted, because function
do_fb_ioctl already returns -ENOTTY when fb_ioctl is not assigned.

Signed-off-by: Stefan Christ <s.christ@phytec.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1469622270-10803-1-git-send-email-s.christ@phytec.de