drm/ttm: Remove the CAP_SYS_ADMIN requirement for bo pinning

This breaks vmwgfx non-root EGL clients and is a remnant from the
TTM user-space interface. This test should be done in the driver.
Replace the remaining placement test with a BUG_ON, since triggering
it is a driver bug.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>