drm/ttm: stop dangerous caching attribute change

When we swapout/in a BO we try to change the caching
attributes of the pages before/after doing the copy.

On x86 this is done by calling set_pages_uc(),
set_memory_wc() or set_pages_wb() for not highmem pages
to update the linear mapping of the page.

On all other platforms we do exactly nothing.

Now on x86 this is unnecessary because copy_highpage() will
either create a temporary mapping of the page which is wb
anyway and destroyed immediately again or use the linear
mapping with the correct caching attributes.

So stop this nonsense and just keep the caching as it is and
return an error when a driver tries to change the caching of
an already populated TT object.

This is much more defensive since changing caching
attributes is platform and driver specific and usually
doesn't work after the page was initially allocated.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/391293/