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

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

drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c: In function
‘amdgpu_amdkfd_evict_userptr’:
drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c:1665:6: warning:
variable ‘invalid’ set but not used [-Wunused-but-set-variable]

'invalid' is never used, so can be removed. Thus 'atomic_inc_return'
can be replaced as 'atomic_inc'

Fixes: 5ae0283e ("drm/amdgpu: Add userptr support for KFD")
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>