drm/i915: check for strange pfit pipe assignemnt on ivb/hsw

drm/i915: check for strange pfit pipe assignemnt on ivb/hsw

Panel fitters on ivb/hsw are not created equal since not all of them
support the new high-quality upscaling mode. To offset this the hw
allows us to freely assign the pfits to pipes.

Since our code currently doesn't support this we might fall over when
taking over firmware state. So check for this case and WARN about it.
We can then improve the code once we've hit this in the wild. Or once
we decide to support the improved upscale modes, though that requires
global arbitrage of modeset resources across crtcs.

v2: Check for IS_GEN7 instead of IS_IVB || IS_HSW as suggested by
Paulo in his review comment.

Suggested-by: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>