drm/amdgpu/gfx: Fix gfx_v7_0_get_csb_buffer to use rb_config

Instead of having the hardcoded values for the CSB buffer in
gfx_v7_0_get_csb_buffer, use the values calculated in previous steps by
accessing raster_config and raster_config_1.

Signed-off-by: Rodrigo Siqueira <siqueira@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>