drm/ast: add AST 2400 support.

This is ported from the userspace driver.

Untested on any ast2400 hw so far.

Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --cc drivers/gpu/drm/ast/ast_drv.h
Simple merge

diff --cc drivers/gpu/drm/ast/ast_main.c
Simple merge

diff --cc drivers/gpu/drm/ast/ast_mode.c
Simple merge

diff --cc drivers/gpu/drm/ast/ast_post.c
Simple merge