drm/exynos: mixer: remove src offset from mixer_graph_buffer()

We always translate the dma address such that the offsets of
the source image are zero. Hence we can remove manipulation of
the MXR_GRAPHIC_SXY(win) register and just zero them once
in mixer_win_reset().

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>