xrandr: Use floating point for VTotal when calculating refresh rate

Interlaced modes generally have an odd VTotal, so we lose half a line
from VTotal when we divide by two. That causes the final refresh rate
to be slightly off. Make VTotal a double to avoid the problem.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>