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 <zack.rusin@broadcom.com>
Fixes: 8b7de6aa ("vmwgfx: Rework fence event action")
Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-23566
Cc: David Airlie <airlied@gmail.com>
CC: Daniel Vetter <daniel@ffwll.ch>
Cc: Zack Rusin <zack.rusin@broadcom.com>
Cc: Broadcom internal kernel review list <bcm-kernel-feedback-list@broadcom.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kernel@vger.kernel.org
Cc: <stable@vger.kernel.org> # v3.4+
Reviewed-by: Maaz Mombasawala <maaz.mombasawala@broadcom.com>
Reviewed-by: Martin Krastev <martin.krastev@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240425192748.1761522-1-zack.rusin@broadcom.com