drm/vboxvideo: create blend mode property on planes

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. Both the vboxvideo primary and cursor planes expose ARGB8888
and trip this on driver load.

VirtualBox draws the cursor through the host windowing system, which
treats the guest-supplied pointer shape as straight
(non-pre-multiplied) alpha: the host frontend loads the pixels
verbatim into an unpremultiplied ARGB image before handing them to the
host cursor APIs. This corresponds to DRM_MODE_BLEND_COVERAGE. Expose
a "pixel blend mode" property advertising only DRM_MODE_BLEND_COVERAGE
to make these semantics explicit and silence the warning. The primary
plane's alpha channel is ignored by the host (opaque blit) and it is
the bottom-most plane anyway; advertise the same value there for
consistency.

Fixes: 860e748bddcc ("drm: ensure blend mode supported if pixel format with alpha exposed")
Acked-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/20260901083234.1828755-5-qinyuntan@linux.alibaba.com

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