drm/i915/vrr: Split vrr-compute-config in two phases

As vrr guardband calculation is dependent on modified
vblank start so better to compute late after all
vblank adjustement.

v1: Initial version.
v2: Split in a separate patch from panel-replay workaround. [Ankit]
v3: Add a function for late vrr related computation. [Ville]
v4: Use flipline instead of vrr.enable and some cosmetic changes. [Ville]
v5: Use intel_vrr_possible helper.

Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
[vsyrjala: Make adjusted_mode const]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241010040503.1795399-3-mitulkumar.ajitkumar.golani@intel.com