drm/msm/sde: add helper macro for performing ktime comparisons

Linux ktime events can not be directly compared if the timer
has overflowed between them. This change adds a helper macro
to first take the difference between the events and then
compare the result against zero in an attempt to handle timer
overflow situations.

Change-Id: Ic32afc35fbabb757714b5a583cc384389176b239
Signed-off-by: Clarence Ip <cip@codeaurora.org>