drm/i915: Reorganize FBC function pointer initializaition

Initialize the FBC vfuncs on gen2 and gen3 chipsets. Also make
a clean split for gen7+ vs. gen5+ vfunc initialization.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>