drm/arc: Inline arcpgu_drm_hdmi_init

Really not worth the function, much less the separate file now that
almost all the code is gone.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210112084358.2771527-12-daniel.vetter@ffwll.ch