drm: Copy back ioctl data to userspace regardless of return code.

Fixes a regression from commit 9d5b3ffc
('drm: fixup some of the ioctl function exit paths'): The vblank ioctl
needs to update the userspace parameters when interrupted by a signal,
which was prevented by the return code check. This could cause the X
server to hang in drmWaitVBlank().

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>