drm/i915: fix SERR_INT init/reset code

drm/i915: fix SERR_INT init/reset code

The SERR_INT register is very similar to the other IIR registers, so
let's zero it at preinstall/uninstall and WARN for a non-zero value at
postinstall, just like we do with the other IIR registers. For this
one, there's no need to double-clear since it can't store more than
one interrupt.

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>