drm/amdgpu/gfx: Use CSB helpers in gfx_v11_0_get_csb_buffer

Part of the code in gfx_v11_0_get_csb_buffer can be removed in favor of
some GFX CSB helpers. This commit removes the duplicated part for the
GFX 11 CSB function.

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