drm/msm/sde: prevent null pointer access on disabled encoder

Check for valid pointers before dereferencing in the encoder's
frame done callback.  This can happen in the rare case when an
encoder is disabled and an overflow interrupt fires for that INTF.

Change-Id: I75273a22f88624b15dd63973adf4a7ac0fb601e7
Signed-off-by: Steve Cohen <cohens@codeaurora.org>