drm/vkms: allow full alpha blending on all planes

Before commit bc0d7fde ("drm: vkms: Supports to the case where
primary plane doesn't match the CRTC"), the composition was executed on
top of the primary plane. Therefore, the primary plane was not able to
support the alpha channel. After commit bc0d7fde, this is possible,
as the composition is now executed on top of the CRTC.

So, allow all planes to support the alpha channel, making full alpha
blending possible in vkms.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Arthur Grillo <arthurgrillo@riseup.net>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230420232228.273340-1-mcanal@igalia.com