drm/i915/vrr: Extract helpers to convert between guardband and pipeline_full values

drm/i915/vrr: Extract helpers to convert between guardband and pipeline_full values

I'd like to move towards a world where we can't more or less
pretend that the ICl/TGL VRR hardware works the same way as
ADL+. To that end extract some helpers to convert between
the guardband and pipeline_full representations.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250917203446.14374-2-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>