drm/exynos: Implement mmap as GEM object function

Moving the driver-specific mmap code into a GEM object function allows
for using DRM helpers for various mmap callbacks.

The respective exynos functions are being removed. The file_operations
structure exynos_drm_driver_fops is now being created by the helper macro
DEFINE_DRM_GEM_FOPS().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixed merge conflict.
Signed-off-by: Inki Dae <inki.dae@samsung.com>