drm/i915/dp: Cache EDID for a detection cycle

As we may query the edid multiple times following a detect, record the
EDID found during output discovery and reuse it. This is a separate
issue from caching the output EDID across detection cycles.

v2: Implement connector->force() callback so that edid is associated
with the connector for user overrides as well (Ville)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>