drm/exynos: g2d: let exynos_g2d_get_ver_ioctl fail

Currently the DRM_IOCTL_EXYNOS_G2D_GET_VER ioctl always succeeds, even
if no G2D support is available. Let the ioctl fail when this is the
case, so that userspace can accurately probe for G2D support.

This also fixes the exynos tests in libdrm. There 'g2d_init' doesn't
fail when G2D is absent, leading to a segfault later.

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