drm/amdgpu: remove RB1_BUSY bit checking

This is a workaround to let VCE soft reset work.
RB1_BUSY bit is always set, so remove its checking now, and we
will depend on RB0_BUSY currently.
After we find the root cause of RB1_BUSY, we can add it back.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>