drm/radeon: fix the crash in benchmark functions

radeon_copy_dma and radeon_copy_blit must be called with
a valid reservation object. Otherwise a crash will be provoked.
We borrow the object from destination BO.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=88464



Cc: stable@vger.kernel.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>