drm/i915:vrr: Separate out functions to compute vmin and vmax

Make helpers to compute vmin and vmax.

v2: Make the adjusted mode const (Ville)
Use reverse xmas tree order of declarations. (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250311093751.1329043-3-ankit.k.nautiyal@intel.com