drm/fb_helper: set info->par in fill_info()

The fbdev emulation helpers pretty much assume that this is set.
Let's do it for everyone.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326132008.11781-3-daniel.vetter@ffwll.ch