i830_disable_pipe() gets called from the power well code, and thus
we're already holding the power domain mutex. That means we can't
call plane->get_hw_state() as it will also try to grab the
same mutex and will thus deadlock.

Replace the assert_plane() calls (which calls ->get_hw_state()) with
just raw register reads in i830_disable_pipe(). As a bonus we can
now get a warning if plane C is enabled even though we don't even
expose it as a drm plane.

v2: Do a separate WARN_ON() for each plane (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Fixes: 51f5a096 ("drm/i915: Add .get_hw_state() method for planes")
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171129125411.29055-1-ville.syrjala@linux.intel.com