drm/i915: Prepare the mode readout for hw vs. uapi state split

Prepare the mode readout for the uapi vs. hw state split.
We'll want to do all readout into the hw state.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927131432.15978-4-ville.syrjala@linux.intel.com