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

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

drivers/gpu/drm/amd/amdkfd/kfd_device.c: In function
‘kgd2kfd_post_reset’:
drivers/gpu/drm/amd/amdkfd/kfd_device.c:745:11: warning:
variable ‘count’ set but not used [-Wunused-but-set-variable]

'count' is never used, so can be removed. Thus 'atomic_dec_return'
can be replaced as 'atomic_dec'

Fixes: e42051d2 ("drm/amdkfd: Implement GPU reset handlers in KFD")
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>