disp: msm: sde: do DRM encoder NULL check before waking up display

DRM encoder can be NULL during modeset concurrency, so add this
change to check drm encoder and only wake up display when drm encoder
is available.

Change-Id: I50dd85eb39567aba4895dc19801020d7ead841b8
Signed-off-by: Lei Chen <chenlei@codeaurora.org>