drm_hwcomposer: Fix EINTR handling in clock_nanosleep

clock_nanosleep return errno directly
instead of -1 with errno set

Signed-off-by: Keith Mok <keithmok@google.com>
[jstultz: Fixed minor style issues]
Signed-off-by: John Stultz <jstultz@google.com>
Change-Id: I779f2a8234b3dae46d64efee2210b7a2c5a92043