drm/vkms: drop full alpha blending TODO

Now that VKMS supports full alpha blending on all planes, drop the
"ARGB format on primary plane" and "Full alpha blending on all planes"
tasks from the TODO list.

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/20230420232228.273340-2-mcanal@igalia.com