drm/ast: Move VRAM size detection to ast_mm.c

VRAM size detection is only relevant to the memory management. Move
the code into ast_mm.c.

While at it, rename the function to ast_get_vram_size(). The function
argument's type is now struct ast_private. The result is stored in a
local variable and not in struct ast_private any longer.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716125353.31512-5-tzimmermann@suse.de