grub: really disable splash screen support

When BR2_TARGET_GRUB_SPLASH is enabled, we were passing
--enable-graphics, but when it was disabled, we were not doing
anything. However, in Grub, graphics support is enabled by default,
and you have to pass --disable-graphics to disable it.

Fix this by passing --enable-graphics or --disable-graphics as
appropriate.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>