drm/i915: Create generic intel_panel for LVDS and eDP

Create a generic struct intel_panel for sharing a data structure and code
between eDP and LVDS panels. Add the new struct to intel_connector so that
later on we can have generic EDID and mode reading functions with EDID
caching that transparently fallback to fixed mode when EDID is not
available.

Add intel_panel as a dummy first, and move data (such as the mentioned
fixed mode) to it in later patches.

Based on earlier work by Chris Wilson <chris@chris-wilson.co.uk>

CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Fixup tiny conflict in intel_dp_destroy.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>