drm/vmwgfx: Don't impose STDU limits on framebuffer size

If framebuffers are larger, we create bounce surfaces that are within
STDU limits.

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>