drm/edid: abstract override/firmware EDID retrieval

Abstract the debugfs override and the firmware EDID retrieval
function. We'll be needing it in the follow-up. No functional changes.

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Harish Chegondi <harish.chegondi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Tested-by: Paul Wise <pabs3@bonedaddy.net>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190607110513.12072-1-jani.nikula@intel.com