drm/amdkfd: fix loop error handling

Clang static analysis reports this problem
kfd_chardev.c:2594:16: warning: The expression is an uninitialized value.
  The computed value will also be garbage
        while (ret && i--) {
                      ^~~

i is a loop variable and this block unwinds a problem in the loop.
When the error happens before the loop, this value is garbage.
Move the initialization of i to its decalaration.

Fixes: be072b06 ("drm/amdkfd: CRIU export BOs as prime dmabuf objects")
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>