staging: vboxvideo: Fix IRQs no longer working

Commit 1daddbc8 ("staging: vboxvideo: Update driver to use
drm_dev_register.") replaced the obsolere drm_get_pci_dev() with
normal pci probe and remove functions.

But the new vbox_pci_probe() is missing a pci_enable_device() call,
causing interrupts to not be delivered. This causes resizes of the
vm window to not get seen by the drm/kms code.

This commit adds the missing pci_enable_device() call, fixing this.

Fixes: 1daddbc8 ("staging: vboxvideo: Update driver to use ...")
Cc: Fabio Rafael da Rosa <fdr@pid42.net>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Nicholas Mc Guire <der.herr@hofr.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>