i915, fbdev: Fix Kconfig typo

Too many t's.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>