drm/amdgpu: remove set but not used variable 'grbm_soft_reset'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c: In function 'gfx_v8_0_pre_soft_reset':
drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c:4950:27: warning:
 variable 'srbm_soft_reset' set but not used [-Wunused-but-set-variable]

drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c: In function 'gfx_v8_0_post_soft_reset':
drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c:5054:27: warning:
 variable 'srbm_soft_reset' set but not used [-Wunused-but-set-variable]

It never used since introduction in commit d31a501e ("drm/amdgpu: add
pre_soft_reset ip func") and e4ae0fc3 ("drm/amdgpu: implement
gfx8 post_soft_reset")
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c

@@ -4952,14 +4952,13 @@ static bool gfx_v8_0_check_soft_reset(void *handle)
 static int gfx_v8_0_pre_soft_reset(void *handle)
 {
 	struct amdgpu_device *adev = (struct amdgpu_device *)handle;
-	u32 grbm_soft_reset = 0, srbm_soft_reset = 0;
+	u32 grbm_soft_reset = 0;
 
 	if ((!adev->gfx.grbm_soft_reset) &&
 	    (!adev->gfx.srbm_soft_reset))
 		return 0;
 
 	grbm_soft_reset = adev->gfx.grbm_soft_reset;
-	srbm_soft_reset = adev->gfx.srbm_soft_reset;
 
 	/* stop the rlc */
 	adev->gfx.rlc.funcs->stop(adev);
@@ -5056,14 +5055,13 @@ static int gfx_v8_0_soft_reset(void *handle)
 static int gfx_v8_0_post_soft_reset(void *handle)
 {
 	struct amdgpu_device *adev = (struct amdgpu_device *)handle;
-	u32 grbm_soft_reset = 0, srbm_soft_reset = 0;
+	u32 grbm_soft_reset = 0;
 
 	if ((!adev->gfx.grbm_soft_reset) &&
 	    (!adev->gfx.srbm_soft_reset))
 		return 0;
 
 	grbm_soft_reset = adev->gfx.grbm_soft_reset;
-	srbm_soft_reset = adev->gfx.srbm_soft_reset;
 
 	if (REG_GET_FIELD(grbm_soft_reset, GRBM_SOFT_RESET, SOFT_RESET_CP) ||
 	    REG_GET_FIELD(grbm_soft_reset, GRBM_SOFT_RESET, SOFT_RESET_CPF) ||