drm/i915: migrate i9xx plane get config

Migrate this code out like the skylake code.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c003bd458a6bcc703e9e2fb05731fb7124012e8c.1612536383.git.jani.nikula@intel.com