drm/msm: fix timeout calculation

The 'timeout' value comes from userspace (CLOCK_MONOTONIC), but
converting this directly to jiffies doesn't take into account the
initial jiffies count at boot, which may differ from the base time
of CLOCK_MONOTONIC.

TODO: add ktime_delta_jiffies() when rebasing on 4.1 and use that
instead of ktime_sub/ktime_to_timespec/timespec_to_jiffies combo (as
suggested by Arnd)

v2: switch over from 'struct timespec' to ktime_t throughout, since
'struct timespec' will be deprecated (as suggested by Arnd)
v3: minor cosmetic tweaks

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Clark <robdclark@gmail.com>