drm/i915: Enable FBC at Haswell.

This patch introduce Frame Buffer Compression (FBC) support for HSW.
FBC is tied to primary plane A in HSW.

v2: Ville pointed out docs say FBC must be disabled before disabling
    the plane on HSW.
v3: Really enabling it by default at HSW.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>