drm/amd/pm: initialize variable

Static analysis reports this problem

amdgpu_pm.c:478:16: warning: The right operand of '<' is a garbage value
  for (i = 0; i < data.nums; i++) {
                ^ ~~~~~~~~~

In some cases data is not set.  Initialize to 0 and flag not setting
data as an error with the existing check.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>