drm/i915: Parse VRR capability from VBT

VBT seems to have an extra flag for VRR vs. not. Let's consult
that for eDP panels.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220531191844.11313-2-ville.syrjala@linux.intel.com