drm/i915: Enable FBC for all mobile gen2 and gen3 platforms

All mobile gen2 and gen3 chipsets should have FBC1, and the code
should now handle them all. So just set has_fbc=true for all such
chipsets.

Note that fbc is still disabled by default for now.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>