drm/virtio: create blend mode property on cursor plane

Since commit 860e748bddcc ("drm: ensure blend mode supported if pixel
format with alpha exposed"), drm_mode_config_validate() warns when a
plane exposes an alpha pixel format but not the "pixel blend mode"
property. The virtio-gpu cursor plane (HOST_ARGB8888) trips this.

The virtio-gpu specification does not define the cursor alpha
semantics. The host forwards the cursor pixels verbatim to its display
frontends, and the remote cursor protocols among them (SPICE alpha
cursors, the VNC "Cursor With Alpha" encoding) both define
pre-multiplied alpha, matching what userspace has always assumed when
the property is not attached. Expose a "pixel blend mode" property
advertising only DRM_MODE_BLEND_PREMULTI to make these semantics
explicit and silence the warning. The primary plane only exposes
HOST_XRGB8888, so the call is gated to the cursor. No functional
change.

Fixes: 860e748bddcc ("drm: ensure blend mode supported if pixel format with alpha exposed")
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Qinyun Tan <qinyuntan@linux.alibaba.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260901083234.1828755-4-qinyuntan@linux.alibaba.com

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