drm/i915: GEN6_RP_INTERRUPT_LIMITS doesn't seem to exist on VLV

I can't find GEN6_RP_INTERRUPT_LIMITS (0xA014) anywhere in VLV docs.
Reading it always returns zero from what I can tell, and eliminating
it doesn't seem to make any difference to the behaviour of the system.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>