drm/radeon: make lockup timeout a module param

Don't hard code the 10 seconds timeout. Compute jobs
can run much longer.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>