drm/amdgpu: remove unused variable num_xcc

gcc with W=1 reports
drivers/gpu/drm/amd/amdgpu/gfx_v9_4_3.c:2138:13: error: variable
  ‘num_xcc’ set but not used [-Werror=unused-but-set-variable]
 2138 |         int num_xcc;
      |             ^~~~~~~

This variable is not used so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>