drm/i915: Fix FBC1 plane checks for gen2

On gen2 and gen3 chipsets FBC is supported only on plane A. Fix (and
simplify) the plane checks in intel_update_fbc() accordingly.

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>