drm/vkms: Add functions to map/unmap GEM backing storage

This patch add the necessary functions to map/unmap GEM
backing memory to the kernel's virtual address space.

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/4b6563ae4f4337a5fd51f872424addf64e8d59a6.1532446182.git.hamohammed.sa@gmail.com