drm/exynos: Don't cast GEM object to Exynos GEM object when not needed

The exynos_drm_gem_dumb_map_offset() doesn't need to access any
Exynos-specific GEM object fields, don't cast the GEM object.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>