omapfb: allow compilation only if DRM_OMAP is disabled

At the moment omapfb and omapdrm can be compiled at the same time, if
both are modules. However, they can't be both loaded, as they use the
same hardware. This has been mostly for compile testing.

To make it clear that omapfb and omapdrm are mutually exclusive drivers,
this patch makes omapfb available only if omapdrm is disabled.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Acked-by: Rob Clark <robdclark@gmail.com>