drm/i915: Don't hardcode the number of pipes in the error state dump

New-ish devices have 3 pipes, so let's not just hardcode 2 but use the
for_each_pipe() macro and make struct intel_display_error_state is big
enough.

V2: Also add the number of pipes emitted (Chris Wilson)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>