amdgpu: stop reading CC_RB_BACKEND_DISABLE on Vega10

Follow up to 'drm: don't access deprecated register on Vega10'.

The same information is available in enabled_rb_pipes_mask and reading that
register can cause GRBM bus problems.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>