drm/msm/sde: fix video encoder access in phy encoder

Video encoder structure member should be accessed
after physical encoder null check. This patch fixes
the current invalid accesses.

Change-Id: Icbd2e5d188a5a3ad459d13a3381325d920cac998
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>