drm/i915: move fbc_mask to runtime info

If it's modified runtime, it's runtime info.

v2: Rebase on mtl fbc_mask

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Maarten Lankhort <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/dd1898084b732ba265b212ddbc0fcdd826c11820.1660910433.git.jani.nikula@intel.com