drm/i915/gt: Add MCR-specific workaround initializers

Let's be more explicit about which of our workarounds are updating MCR
registers.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221014230239.1023689-12-matthew.d.roper@intel.com