drm/i915: Extract intel_mode_vblank_start()

We want to calculate the vblank_start for vblank evasion
differently for vrr. To make that nicer lets first extract
the current non-vrr case to a helper.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210122232647.22688-4-manasi.d.navare@intel.com