drm/i915/pmu: Fix uninitialized variable on error path

If name allocation failed the log message will contain an uninitialized
error code which can be confusing.

Fixes: 05488673 ("drm/i915/pmu: Support multiple GPUs")
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191018090514.1818-1-tvrtko.ursulin@linux.intel.com
[tursulin: Commit message spelling fix.]