drm/ttm: nuke VM_MIXEDMAP on BO mappings v3

drm/ttm: nuke VM_MIXEDMAP on BO mappings v3

We discussed if that is really the right approach for quite a while now, but
digging deeper into a bug report on arm turned out that this is actually
horrible broken right now.

The reason for this is that vmf_insert_mixed_prot() always tries to grab
a reference to the underlaying page on architectures without
ARCH_HAS_PTE_SPECIAL and as far as I can see also enabled GUP.

So nuke using VM_MIXEDMAP here and use VM_PFNMAP instead.

Also make sure to reject mappings without VM_SHARED.

v2: reject COW mappings, merge function with only caller
v3: adjust comment as suggested by Thomas

Signed-off-by: Christian König <[email protected]>
Bugs: https://gitlab.freedesktop.org/drm/amd/-/issues/1606#note_936174
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Thomas Hellström <[email protected]>