drm/i915: disable RCBP and VDS unit clock gating on SNB and VLV

The RCBP workaround still applies on these chips, and we need VDS as well.

v2: remove MB boot fetch that snuck in (Daniel)
    add workaround tags to comments for easier internal tracking (Daniel)
v3: only apply RCPB and VDS on SNB and VLV, IVB doesn't need them (Eugeni)

References: https://bugs.freedesktop.org/show_bug.cgi?id=50251


Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>