drm/bochs: fix bochs_gem_prime_mmap

ttm_fbdev_mmap() just doesn't work.  It appears to work fine, mmap()
returns success, but any attempt to actually access the mapping causes a
SIGBUS.

We can just use drm_gem_prime_mmap() instead.  Almost.  We have to copy
over the start offset from the ttm_buffer_object vm_node to the
drm_gem_object vm_node so the offset math in drm_gem_prime_mmap() works
correctly for us even though we use ttm to manage our objects.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190204183858.8976-1-kraxel@redhat.com