drm/i915/fbc: Parametrize FBC register offsets

Parametrize ilk+ FBC register offsets based on the FBC instance.

v2: More intel_ namespace (Jani)
v3: Don't break gvt (Jani)

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/20211214184616.1410-1-ville.syrjala@linux.intel.com