drm/nouveau/dispnv50: Don't call drm_atomic_get_crtc_state() in prepare_fb

drm/nouveau/dispnv50: Don't call drm_atomic_get_crtc_state() in prepare_fb

Since we recently started warning about uses of this function after the
atomic check phase completes, we've started getting warnings about this in
nouveau. It appears a misplaced drm_atomic_get_crtc_state() call has been
hiding in our .prepare_fb callback for a while.

So, fix this by adding a new nv50_head_atom_get_new() function and use that
in our .prepare_fb callback instead.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Fixes: 1590700d94ac ("drm/nouveau/kms/nv50-: split each resource type into their own source files")
Cc: <stable@vger.kernel.org> # v4.18+
Link: https://patch.msgid.link/20251211190256.396742-1-lyude@redhat.com