drm/i915: Stop gathering error states for CS error interrupts

There's quite a few bug reports with error states where the error
reasons makes just about no sense at all. Like dying on tlbs for a
display plane that's not even there. Also users don't really report a
lot of bad side effects generally, just the error states.

Furthermore we don't even enable these interrupts any more on gen5+
(though the handling code is still there). So this mostly concerns old
platforms.

Given all that lets make our lives a bit easier and stop capturing
error states, in the hopes that we can just ignore them. In case
that's not true and the gpu indeed dies the hangcheck should
eventually kick in. And I've left some debug log in to make this case
noticeble. Referenced bug is just an example.

v2: Fix missing \n Jani spotted.

References: https://bugs.freedesktop.org/show_bug.cgi?id=82095
References: https://bugs.freedesktop.org/show_bug.cgi?id=85944
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>