drm/vkms: Adjust vkms_state->active_planes allocation type

drm/vkms: Adjust vkms_state->active_planes allocation type

In preparation for making the kmalloc family of allocators type aware,
we need to make sure that the returned type from the allocation matches
the type of the variable being assigned. (Before, the allocator would
always return "void *", which can be implicitly cast to any pointer type.)

The assigned type is "struct vkms_plane_state **", but the returned type
will be "struct drm_plane **". These are the same size (pointer size), but
the types don't match. Adjust the allocation type to match the assignment.

Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Fixes: 8b1865873651 ("drm/vkms: totally reworked crc data tracking")
Link: https://lore.kernel.org/r/20250426061431.work.304-kees@kernel.org
Signed-off-by: Louis Chauvet <contact@louischauvet.fr>