drm/i915: vlv: sanitize RPS interrupt mask during GPU idling

We apply the RPS interrupt workaround on VLV everywhere except when
writing the mask directly during idling the GPU. For consistency do this
also there.

While at it also extend the code comment about affected platforms.
I couldn't reproduce the issue on VLV fixed by this workaround, by
removing the workaround from everywhere, while it's 100% reproducible on
SNB using igt/gem_reset_stats/ban-ctx-render. So also add a note that
it hasn't been verified if the workaround really applies to VLV/CHV.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>