drm/radeon/benchmark: make sure bo blit copy exists before using it

Fixes a segfault on asics without a blit callback.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=62239



Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org