drm: probe_helper: Hide ugly ifdef

Push the ifdef to the drm_edid.h and create a stub, for the
DRM_LOAD_EDID_FIRMWARE=n case. This removes some clutter in
the code, making it more readable.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461087638-16959-1-git-send-email-ezequiel@vanguardiasur.com.ar