drm/msm/sde: get previous inline fbo during atomic check

The intermediate buffer for inline rotation can't be
allocated until the plane's "prepare" callback, but
attempt to acquire a previously allocated intermediate
buffer during the atomic check instead of waiting until
the prepare callback. This holds an extra reference count
on the buffer so that it persists past the end of the
atomic check phase.

CRs-Fixed: 2062021
Change-Id: I28c4a50f3272b41cebcdef05b0aaeabf5dc494b7
Signed-off-by: Clarence Ip <cip@codeaurora.org>