drm/msm: clear interface configuration during encoder disable

During atomic commit for encoder disable, driver needs to clear
the interface configuration inside control path. Otherwise, it
may conflict with interface setting from next encoder enable,
and cause hardware hang.

Change-Id: I3102b745479efc10963b2a884a5a23693a13e4c3
Signed-off-by: Jin Li <jinl@codeaurora.org>