drm/msm/sde: clear vbif errors before kickoff

Log and clear any vbif errors before each encoder kickoff.
This ensures that any reported errors are relevant to the
current commit cycle.

Change-Id: I8c1050acd5d6161d897b974968a5c55f69baed4d
Signed-off-by: Clarence Ip <cip@codeaurora.org>