drm/vmwgfx: Replace deprecated PTR_RET

Commit 508108ea ("drm/vmwgfx: Don't refcount command-buffer managed
resource lookups during command buffer validation") slips in use of
deprecated PTR_RET. Use PTR_ERR_OR_ZERO instead.

As the PTR_ERR_OR_ZERO is a bit longer than PTR_RET, we introduce
local variable ret for proper indentation and line-length limits.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>