drm/i915: Prepare the connector/encoder mask readout for hw vs. uapi state split

Prepare the connector/encoder mask 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/20190927131432.15978-2-ville.syrjala@linux.intel.com