drm/vmwgfx: Fix invalid reads in fence signaled events
In the Linux kernel, the following vulnerability has been resolved:

drm/vmwgfx: Fix invalid reads in fence signaled events

Correctly set the length of the drm_event to the size of the structure
that's actually used.

The length of the drm_event was set to the parent structure instead of
to the drm_vmw_event_fence which is supposed to be read. drm_read
uses the length parameter to copy the event to the user space thus
resuling in oob reads.