drm/ast: Put AST_DRAM_ constants into enum ast_dram_layout

drm/ast: Put AST_DRAM_ constants into enum ast_dram_layout

The AST_DRAM_ constants belong together, so put them in an enum
type. Rename type and variables to 'drm_layout', as there's already
another DRAM type in the ast driver (AST_DDR2, AST_DDR3).

v2:
- avoid compiler warning with switch default (Dan)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250826065032.344412-7-tzimmermann@suse.de