drm/i915: don't set the FBC plane select bits on HSW+

This commit is just to make the intentions explicit: on HSW+ these
bits are MBZ, but since we only support plane A and the macro
evaluates to zero when plane A is the parameter, we're not fixing any
bug.

v2:
 - Remove useless extra blank like (Chris).
 - Init dpfc_ctl in another place (Chris).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>