drm/i915: check for FBC planes in the same place as the pipes

This moves the pre-gen4 check from update() to enable(). The HAS_DDI
in the original code is not needed since only gen 2/3 have the plane
swapping code.

v2: Rebase.
v3: Extract fbc_on_plane_a_only() (Chris).

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/