drm/ttm: remove TTM_MEMTYPE_FLAG_FIXED v2

Instead use a boolean field in the memory manager structure.

Also invert the meaning of the field since the use of a TT
structure is the special case here.

v2: cleanup zero init.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/382079/