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Lyude Paul 554ab79cfd drm/nouveau/disp/r535: Add scanline position support + head state support

That's right! It looks like this never actually got finished, something
which I just noticed today when I saw this fun message spamming one of my
test machine's kernel logs when enabling display debug output for nouveau:

  [drm:drm_crtc_vblank_helper_get_vblank_timestamp_internal] crtc 0 : scanoutpos query failed.

So it looks like we've been falling back to DRM's core fallback for a while
now, whoops.

So, while it seems that we do have the option of doing this through GSP -
that doesn't seem like a great idea. Mainly because reading this from GSP
would involve a lot more latency then we should have for vblank handling
due to the RPC communication. So instead of implementing that, just use
gv100_head_state and gv100_head_rgpos for implementing .state and .rgpos.
It seems to work perfectly fine!

Fixes: 9e99444490 ("drm/nouveau/disp/r535: initial support")
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Timur Tabi <ttabi@nvidia.com>
Cc: Ben Skeggs <bskeggs@nvidia.com>
Cc: James Jones <jajones@nvidia.com>
Cc: Faith Ekstrand <faith.ekstrand@collabora.com>
Cc: Suraj Kandpal <suraj.kandpal@intel.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Aaron Kling <webgeek1234@gmail.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Zhang Enpei <zhang.enpei@zte.com.cn>
Cc: <stable@vger.kernel.org> # v6.7+
Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patch.msgid.link/20260429030348.3930866-1-lyude@redhat.com
(cherry picked from commit 804cb093b2)
Signed-off-by: Lyude Paul <lyude@redhat.com>