drm/i915: check the power domains in ironlake_get_pipe_config()

Just like we already do in haswell_get_pipe_config(). This should
prevent some WARNs when we run pm_rpm on SNB.

Testcase: igt/pm_rpm
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80463


Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>