drm/i915/fbc: Enable fbc on i865

Unlike all the other pre-snb desktop platforms i865 actually
supports FBC. Let's enable it.

Quote from the spec:
"DevSDG provides the same Run-Length Encoded Frame Buffer
 Compression (RLEFBC) function as exists in DevMGM."

As i865 only has the one pipe we want to skip massaging the
plane<->pipe assignment aimed at getting FBC+LVDS working on
the mobile platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702153723.24327-4-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>