drm/msm/sde: don't convert ktime to ns when waiting for fences

The driver should utilize the ktime_ series of functions to directly
manipulate the time structures rather than converting everything to
nanoseconds. This avoids possible trunction/rounding errors that may
be introduced by the conversion process.

Change-Id: Iac3dd40c679ffc9517881f41650aa5848dab884b
Signed-off-by: Clarence Ip <cip@codeaurora.org>