drm/ast: dp501: Fix initialization of SCU2C

drm/ast: dp501: Fix initialization of SCU2C

Ast's DP501 initialization reads the register SCU2C at offset 0x1202c
and tries to set it to source data from VGA. But writes the update to
offset 0x0, with unknown results. Write the result to SCU instead.

The bug only happens in ast_init_analog(). There's similar code in
ast_init_dvo(), which works correctly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 83c6620bae3f ("drm/ast: initial DP501 support (v0.2)")
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v3.16+
Link: https://patch.msgid.link/20260327133532.79696-2-tzimmermann@suse.de