drm/exynos: Use mode->clock instead of reverse calculating it from the vrefresh

drm/exynos: Use mode->clock instead of reverse calculating it from the vrefresh

htotal*vtotal*vrefresh ~= clock. So just use say "clock" when we mean it.

Cc: Inki Dae <inki.dae@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>