drm/amd/amdgpu/gfx_v10_0: Remove a bunch of set but unused variables

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c: In function ‘gfx_v10_rlcg_wreg’:
 drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c:1416:18: warning: variable ‘grbm_idx’ set but not used [-Wunused-but-set-variable]
 drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c:1415:18: warning: variable ‘grbm_cntl’ set but not used [-Wunused-but-set-variable]
 drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c:1413:15: warning: variable ‘scratch_reg3’ set but not used [-Wunused-but-set-variable]
 drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c:1412:15: warning: variable ‘scratch_reg2’ set but not used [-Wunused-but-set-variable]

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>