fbdev: radeon: use ktime_get() for HZ calibration

do_gettimeofday() is deprecated and a bit clumsy. This changes
radeon_probe_pll_params() over to using ktime_get() with monotonic
times. There is no need to check for negative values any more
since the monotonic clocksource cannot go backwards, but I'm
adding a check for zero-division in case of a bad clocksource.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>