drm/msm/sde: send start command to inline rotation

Inline rotations are handled by the external rotator driver,
but any timeout detection by the rotator needs to be delayed
until after the DRM is ready to start the commit operation.

The rotator is notified during the plane's flush function
that the DRM is ready to start the hardware commit.

CRs-Fixed: 2046958
Change-Id: I9c1a56cbe0b0fe4a07b1cbcddccc4f62c83ccdcf
Signed-off-by: Clarence Ip <cip@codeaurora.org>