drm/i915/fbc: Introduce device info fbc_mask

Declare which FBC instances are present via a fbc_mask
in device info. For the moment there is just the one.

TODO: Need to figure out how to expose multiple FBC
instances in debugs. Just different file names, or move
the files under some subdirectory (per-crtc maybe), or
something else? This will need igt changes as well.

v2: Put the mask into device_info.display (Jani)
    Put the magic pipe->fbc thing into skl_fbc_id_for_pipe() (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/20211213134450.3082-4-ville.syrjala@linux.intel.com