drm/omapdrm: dispc: Refuse x-decimation above 4 for all but 8-bit formats

Let's disable all scaling that requires horizontal decimation with
higher factor than 4, until we have better estimates of what we can
and can not do. However, NV12 color format appears to work Ok with
all decimation factors.

When decimating horizontally by more that 4 the dss is not able to
fetch the data in burst mode. When this happens it is hard to tell if
there enough bandwidth. Despite what theory says this appears to be
true also for 16-bit color formats.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>