CHROMIUM: drm/virtio: Support mmap with non zero offset

mmap() with non zero offset fails on Chromium OS VM because
ttm_fbdev_mmap() returns -EACCESS if the offset is not zero. To aoivd this
issue, offset is set to 0 and updated after ttm_fbdev_mmap().

BUG=chromium:911370, chromium:923061,
TEST=video.WebRTCCamera on VM

Change-Id: I1da60c6e0418505ae31770820cac868b1faf906f
Signed-off-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1429921
Reviewed-by: Tomasz Figa <tfiga@chromium.org>