drm/radeon: Remove tests for -ERESTART from the TTM code.

Also sets affected TTM calls up to not wait interruptible, since
that would cause an in-kernel spin until the TTM call succeeds, since
the Radeon code does not return to user-space when a signal is received.

Modifies interruptible fence waits to return -ERESTARTSYS rather than
-EBUSY when interrupted by a signal, since that's the (yet undocumented)
semantics required by the TTM sync object hooks.

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