drm/amdkfd: Fix signal_eviction_fence() bool return value

signal_eviction_fence() is declared to return bool, but returns -EINVAL
when no eviction fence is present. This makes the "no fence" or "the
NULL-fence" path evaluate to true and triggers a Smatch warning.

v2: Return true instead to explicitly indicate that there is no eviction
fence to signal and that eviction is already complete. This matches the
existing caller logic where a NULL fence means "nothing to do" and
allows restore handling to proceed normally. (Christian)

Fixes the below:
drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_process.c:2099 signal_eviction_fence()
warn: '(-22)' is not bool

drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_process.c
2090 static bool signal_eviction_fence(struct kfd_process *p)
^^^^

2091 {
2092 struct dma_fence *ef;
2093 bool ret;
2094
2095 rcu_read_lock();
2096 ef = dma_fence_get_rcu_safe(&p->ef);
2097 rcu_read_unlock();
2098 if (!ef)
--> 2099 return -EINVAL;

This should be either true or false.
Probably true because presumably
it has been tested?

2100
2101 ret = dma_fence_check_and_signal(ef);
2102 dma_fence_put(ef);
2103
2104 return ret;
2105 }

Fixes: 37865e02e6cc ("drm/amdkfd: Fix eviction fence handling")
Reported by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Philip Yang <Philip.Yang@amd.com>
Cc: Gang BA <Gang.Ba@amd.com>
Cc: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>