drm/i915: add L3 bank clock gating disable on VLV

Prevents a possible hang: WaDisableL3Bank2xClockGate.

v2: only apply to VLV, IVB doesn't need this anymore

References: https://bugs.freedesktop.org/show_bug.cgi?id=50245


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>