drm/vmwgfx: Fix invalid kref_put callback in vmw_bo_dirty_release

drm/vmwgfx: Fix invalid kref_put callback in vmw_bo_dirty_release

The kref_put() call uses (void *)kvfree as the release callback, which
is incorrect. kref_put() expects a function with signature
void (*release)(struct kref *), but kvfree has signature
void (*)(const void *). Calling through an incompatible function pointer
is undefined behavior.

The code only worked by accident because ref_count is the first member
of vmw_bo_dirty, making the kref pointer equal to the struct pointer.

Fix this by adding a proper release callback that uses container_of()
to retrieve the containing structure before freeing.

Fixes: c1962742ffff ("drm/vmwgfx: Use kref in vmw_bo_dirty")
Signed-off-by: Brad Spengler <brad.spengler@opensrcsec.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Cc: Ian Forbes <ian.forbes@broadcom.com>
Link: https://patch.msgid.link/20260107171236.3573118-1-zack.rusin@broadcom.com