drm/i915: Add support for FBC on Ivybridge.

This patch introduce Frame Buffer Compression (FBC) support for IVB,
without enabling it by default.
It adds a new function gen7_enable_fbc to avoid getting
ironlake_enable_fbc messed with many IS_IVYBRIDGE checks.

v2: Fixes from Ville.
     	*  Fix Plane. FBC is tied to primary plane A in HSW
    	*  Fix DPFC initial write to avoid let trash on the register.
v3: Checking for bad plane on intel_update_fbc() as Chris suggested.
v4: Ville pointed out that according to BSpec FBC_CTL bits 0:3 must be 0.
v5: Up to v4 this work was entirely focused on Haswell. However Ville
    noticed I could reuse the FBC work done for HSW and get FBC for free
    at Ivybridge. So it makes more sense enable FBC for IVB first.
    FBC for HSW comming on next patches. We are just not enabling it by
    default on IVB.
v6: Fix confused commit name (by Matt Turner).
v7: Remove gtt_offset shift since it is page aligned byte offset (by Ville).

Cc: Matt Turner <mattst88@gmail.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>