drm/radeon: make ib size variable

This avoid to waste ib pool size and avoid a bunch of wait for
previous ib to finish.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>