drm: Fix missing unlock and free on error in drm_legacy_addbufs_pci()

Add the missing unlock and free before return from function
drm_legacy_addbufs_pci() in the error handling case.

Fixes: 70556e24 ("drm: remove usage of drm_pci_alloc/free")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1621341302-112089-1-git-send-email-zou_wei@huawei.com