exynos: fimg2d: fix return codes

Even if flushing the command buffer doesn't succeed, the
G2D calls would still return zero. Fix this by just passing
the flush return code.

In fact error handling currently ignores the fact that
g2d_add_cmd() can fail. This is going to be handled
in a later patch.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>