drm/sti: Use 64-bit timestamps

'struct timespec' uses a 32-bit field for seconds, which
will overflow in year 2038 and beyond. This patch is part
of a larger attempt to remove instances of timeval, timespec
and time_t, all of which suffer from the y2038 issue, from the
kernel.

Signed-off-by: Tina Ruchandani <ruchandani.tina@gmail.com>