If we need to force a full plane update before userspace/fbdev
have given us a proper plane state we should try to maintain the
current plane state as much as possible (apart from the parts
of the state we're trying to fix up with the plane update).
To that end add basic readout for the plane rotation and
maintain it during the initial fb takeover.

Cc: Hans de Goede <hdegoede@redhat.com>
Fixes: 516a49cc ("drm/i915: Fix assert_plane() warning on bootup with external display")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181120135450.3634-2-ville.syrjala@linux.intel.comTested-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>