drm/amdgpu: fix incorrect active rb bitmap for gfx11

GFX v11 changes RB_BACKEND_DISABLE related registers
from per SA to global ones. The approach to query active
rb bitmap needs to be changed accordingly. Query per
SE setting returns wrong active RB bitmap especially
in the case when some of SA are disabled. With the new
approach, driver will generate the active rb bitmap
based on active SA bitmap and global active RB bitmap.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>