drm/vmwgfx: Fix return value check in vmw_setup_pci_resources()

In case of error, the function devm_ioremap() returns NULL pointer not ERR_PTR().
The IS_ERR() test in the return value check should be replaced with NULL test.
After that, the error code -ENOMEM should be returned.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Qiheng Lin <linqiheng@huawei.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210514082812.1697-1-linqiheng@huawei.com