drm/exynos: remove function exynos_drm_gem_map_buf

The exynos_drm_gem_map_buf can be merged in exynos_drm_gem_fault.

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