drm/exynos: gem: Remove dead-code

The ExynosDRM page fault handler is never used, drm_gem_mmap()
always calls exynos_drm_gem_mmap() function, which perform
complete mapping for the given virtual address-space area.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>