drm/vmwgfx: Implement fbdev on kms v2

With screen targets the old legacy display system fbdev doesn't work
satisfactory anymore. At best the resolution is severely restricted.
Therefore implement fbdev on top of the kms system. With this change, fbdev
will be using whatever KMS backend is chosen.

There are helpers available for this, so in the future we'd probably want
to implement the helper callbacks instead of calling into our KMS
implementation directly.

v2: Make sure we take the mode_config mutex around modesetting,
Also clear the initial framebuffer using vzalloc instead of vmalloc.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>