drm/ast: Detect DRAM before TX-chip

Move DRAM detection before TX-chip detection. Both steps are independent
from each other. Detection of the TX-chip is now next to posting those
chips, which can be done in a single step.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250117103450.28692-3-tzimmermann@suse.de