drm/vmwgfx: Delay pinning fbdev framebuffer until after mode set

For the Screen Object display unit, we need to reserve a
guest-invisible region equal to the size of the framebuffer for
the host.  This region can only be reserved in VRAM, whereas
the guest-visible framebuffer can be reserved in either VRAM or
GMR.

As such priority should be given to the guest-invisible
region otherwise in a limited VRAM situation, we can fail to
allocate this region.

This patch makes it so that vmw_sou_backing_alloc() is called
before the framebuffer is pinned.

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Cc: <stable@vger.kernel.org>
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This is the last patch of a 3-patch series to fix console black
screen issue on Ubuntu 16.04 server