drm/exynos: add DRM_EXYNOS_GEM_MAP ioctl

The commit d931589c ("drm/exynos: remove DRM_EXYNOS_GEM_MAP_OFFSET
ioctl") removed it same with the ioctl that this patch adds. The reason
that removed DRM_EXYNOS_GEM_MAP_OFFSET was we could use
DRM_IOCTL_MODE_MAP_DUMB. Both did exactly same thing.

Now we again will revive it as DRM_EXYNOS_GEM_MAP because of render
node. DRM_IOCTL_MODE_MAP_DUMB isn't permitted in render node.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>