drm: Allow override_edid to override the firmware EDID

IMO the override_edid should override any default EDID for the
connector, whether that came in via the connector helper ->get_modes()
vfunc or via the firmware EDID mechanism.

Cc: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449782037-19722-2-git-send-email-ville.syrjala@linux.intel.com