drm/ast: Remove vaddr field from struct ast_plane

The vaddr field in struct ast_plane serves no purpose. Its value
can be calculated easily from the VRAM base plus the plane offset.
Do so and remove the field.

In ast_primary_plane_helper_get_scanout_buffer(), remove the test
for vaddr being NULL. This cannot legally happen.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250324094520.192974-3-tzimmermann@suse.de