drm/fb-helper: Improve error reporting in setup

Improve error reporting in drm_fb_helper_fbdev_setup() by printing the
error code. This is useful for drivers that choose to not fall over just
because fbdev doesen't work, but still wants clues to why it failed.
This way they don't have to provide an error message themselves.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180908134648.2582-2-noralf@tronnes.org