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>