drm/vmwgfx: Fix user space handle equal to zero

User-space handles equal to zero are interpreted as uninitialized or
illegal by some drm systems (most notably kms). This means that a
dumb buffer or surface with a zero user-space handle can never be
used as a kms frame-buffer.

Cc: <stable@vger.kernel.org>
Fixes: c7eae626

 ("drm/vmwgfx: Make the object handles idr-generated")
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>