xf86drm: Fix possible memory leak with drmModeGetPropertyPtr()

In drmModeGetPropertyPtr(), from upper error path, it calls free
but with just next error path, it does not call. Fix the possible
memory leak.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>