drm: Make drm_mode_vrefresh() a bit more accurate

Do the refresh rate calculation with a single division. This gives
us slightly more accurate results, especially for interlaced since
we don't just double the final truncated result.

We do lose one bit compared to the old way, so with an interlaced
mode the new code can only handle ~2GHz instead of the ~4GHz the
old code handeled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180313150759.27620-2-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>