drm/arc: Delete arcpgu_priv->fb

Leftover from the conversion to the generic fbdev emulation.

Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210112084358.2771527-3-daniel.vetter@ffwll.ch