staging: vboxvideo: Fix NULL ptr deref in vbox_set_up_input_mapping()

When vbox_set_up_input_mapping() gets called the first crtc might be
disable and not have a fb at all, triggering a NUL ptr deref at:

			vbox->input_mapping_width = CRTC_FB(crtci)->width;

Instead of using the fb from the crtc with id 0, just use the fb from
the first crtc with a fb. This is in the single_framebuffer = true path,
so all crtc-s point to the same fb anyways.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>