drm/ast: Move DRAM info next to its only user

drm/ast: Move DRAM info next to its only user

The only place in the ast driver that uses the DRAM type is the
P2A DRAM initialization for Gen2 and Gen3 of the chip. Condense
the code in ast_get_dram_info() to exactly this use case and move
it into the Gen's custom source file. Remove the field dram_type
from struct ast_device.

The AST_DRAM_ constants are also used in Gen4 POST helpers, but
independently from the dram_type field. No changes there.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250826065032.344412-6-tzimmermann@suse.de