drm/ast: free correct pointer in astfb_create() error paths

In the err_free_vram and err_release_fbi error paths in astfb_create(), we
attempt to free afbdev->sysram. The only jumps to these error paths occur
before we assign afbdev->sysram = sysram. Free sysram instead.

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114030359.27852-1-andrew.donnellan@au1.ibm.com