drm/i915: fix GEN7_ERR_INT init/reset code

Same as SERR_INT and the other IIR registers: reset on
preinstall/uninstall and WARN for non-zero values at postinstall. This
one also doesn't need double-clear.

v2: - Remove the is_zero assertion (Ben).

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>