drm/ast: Handle known struct ast_dramstruct with helpers

Most of struct ast_dramstruct stores hardware state. Some index
values have known or special meaning. The known values are

- 0xffff - Terminal entry in the array
- 0xff00 - Delays the programming for usecs
- 0x0004 - Sets the type of DRAM

Add constants and helper macros for these cases. Also add a helper
macro for testing. Update Gen1 and Gen2+ accordingly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250706162816.211552-8-tzimmermann@suse.de