exynos/fimg2d: remove unused-function build warning

The function g2d_reset() is not anymore used after the commit
e3c97d1a ("exynos/fimg2d: add g2d_validate_xyz() functions"),
so it should be removed.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Reviewed-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>