msm/drm/sde: handle dynamic mode switch issue

This change checks if input handler is already registered during
dynamic mode switch time. Encoder is not disabled in dynamic mode
switch resulting into input handler regsirtation called twice leading
to error.

Change-Id: Ica21d6be6c874b631f356c689b03ca6465717b81
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>