drm/msm/sde: Correct sequence for input handler flush work

Flush work is done after input handler unregister in encoder virt
disable to avoid race condition. Unused variables in encoder enable
are removed.

Change-Id: Idce3c685deced3ac3d826911ee33e2fc6b6415b4
Signed-off-by: Shubhashree Dhar <dhar@codeaurora.org>