drm/ast: Move Gen6+ POST code to separate source file

Move POST code for Gen6+ to separate source file and hide it in
ast_2500_post(). With P2A configuration, it performs a full board
POST; otherwise it enables the transmitter chip. No changes to the
overall logic.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250706162816.211552-4-tzimmermann@suse.de

drivers/gpu/drm/ast/Makefile

@@ -4,6 +4,7 @@
 # Direct Rendering Infrastructure (DRI) in XFree86 4.1.0 and higher.
 
 ast-y := \
+	ast_2500.o \
 	ast_2600.o \
 	ast_cursor.o \
 	ast_ddc.o \