drm/exynos: fix size check in g2d_check_buf_desc_is_valid()

The size check was incomplete. It only computed the
size of area of the drawing rectangle and checked if
the size still fit inside the buffer.

The correct check is to compute the position of the
last byte that the G2D engine is going to access and
then check if that position is still contained in the
buffer. In particular we need the stride information
to determine this.

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