drm/i915/vrr: Add helper to check if vrr possible

Add helper to check if vrr is possible based on flipline
is computed.

--v1:
return just flipline instead using ternary operator [Jonathan, Ville].

Signed-off-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241010040503.1795399-2-mitulkumar.ajitkumar.golani@intel.com