msm: sde: support inline rotation start event

Inline rotations are triggered by the external DRM driver, so
the rotator driver needs to delay its 'is done' checking until
the DRM driver indicates that the commit has started.

This patch adds support for an 'inline start' command that may
be sent by the DRM driver to indicate that the hardware
commit is ready to begin.

CRs-Fixed: 2046958
Change-Id: I460fde8de1a3b815785f3ae7ea21c986b3ca68f1
Signed-off-by: Clarence Ip <cip@codeaurora.org>