drm/i915/dsi/xelpd: Fix the bit mask for wakeup GB

v2: Fix the typo, move out the hardcoding from
    macro(Jani, Ville)

Fixes: f87c46c4 ("drm/i915/dsi/xelpd: Add WA to program LP to HS wakeup guardband")
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211019151435.20477-2-vandita.kulkarni@intel.com