drm/vram-helper: stop implementing init_mem_type

Instead just initialize the memory type parameters
before calling ttm_bo_init_mm.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/382086/