drm: bridge: Return NULL on error from drm_bridge_get_edid()

The drm_bridge_get_edid() function is documented to return an error
pointer on error. The underlying .get_edid() operation, however, returns
NULL on error, and so do the drm_get_edid() and drm_do_get_edid()
functions upon which .get_edid() is usually implemented. Make
drm_bridge_get_edid() return NULL on error to be consistent.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200526011505.31884-6-laurent.pinchart+renesas@ideasonboard.com