exynos/fimg2d: remove g2d_context from public header

All functions from the public API only operation on
struct g2d_context*, so this shouldn't break too much.

Make the context private since we don't want the
user to modify its content directly. Also remove
the defines that were only used for fields of
g2d_context.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>