drm/amdgpu: Fix unused variable in amdgpu_gfx.c

drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c: In function ‘amdgpu_gfx_disable_kcq’:
drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c:497:6: warning: variable ‘j’ set but not used [-Wunused-but-set-variable]
  497 |  int j;
      |      ^
drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c: In function ‘amdgpu_gfx_disable_kgq’:
drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c:528:6: warning: variable ‘j’ set but not used [-Wunused-but-set-variable]
  528 |  int j;
      |      ^
drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c: In function ‘amdgpu_gfx_enable_kgq’:
drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c:630:12: warning: variable ‘j’ set but not used [-Wunused-but-set-variable]
  630 |  int r, i, j;
      |

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>