drm/vkms: map/unmap buffers in [prepare/cleanup]_fb hooks

This patch map/unmap GEM backing memory to kernel address space
in prepare/cleanup_fb respectively and cache the virtual address
for later use.

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/17d19f61b6539ce1b614c59762d04d816261b307.1532446182.git.hamohammed.sa@gmail.com