drm/i915: Nuke mode.vrefresh usage

drm/i915: Nuke mode.vrefresh usage

mode.vrefresh is rounded to the nearest integer. You don't want to use
it anywhere that requires precision. Also I want to nuke it.
vtotal*vrefresh == 1000*clock/htotal, so let's use the latter.

Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429185457.26235-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>