drm/radeon: disable tests/benchmarks if accel is disabled

commit 4a1132a023eb48cf10522d84c5908d43b612c041 upstream.

The tests are only usable if the acceleration engines have
been successfully initialized.

Based on an initial patch from: Alex Ivanov <gnidorah@p0n4ik.tk>

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[bwh: Backported to 3.2: there is only one test: radeon_test_moves()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>