Export a helper originally added to kms_vrr.c to convert a vertical
refresh rate to frame time. Use uint32_t for the type of vrefresh
matching the type of drmModeModeInfo::vrefresh.

Reviewed-by: Swati Sharma <swati2.sharma@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>