drm/vmwgfx: Fix invalid reads in fence signaled events

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.

Signed-off-by: Zack Rusin <[email protected]>
Fixes: 8b7de6aa8468 ("vmwgfx: Rework fence event action")
Reported-by: [email protected] # ZDI-CAN-23566
Cc: David Airlie <[email protected]>
CC: Daniel Vetter <[email protected]>
Cc: Zack Rusin <[email protected]>
Cc: Broadcom internal kernel review list <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: <[email protected]> # v3.4+
Reviewed-by: Maaz Mombasawala <[email protected]>
Reviewed-by: Martin Krastev <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]