drm/vmwgfx: Remove the deprecated lower mem limit

TTM during the transition to the new page allocator lost the ability
to constrain the allocations via the lower_mem_limit. The code has
been unused since the change:
256dd44b ("drm/ttm: nuke old page allocator")
and there's no reason to keep it.

Fixes: 256dd44b ("drm/ttm: nuke old page allocator")
Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211105193845.258816-2-zackr@vmware.com