drm/amdgpu: clean-up unused variable

Variable 'bp' seems to be unused residue from previous
logic, and is not required anymore.

Cc: Koenig Christian <christian.koenig@amd.com>
Cc: Deucher Alexander <alexander.deucher@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>