drm/vmwgfx: Don't use drm_irq_[un]install

We're not allowed to change the upstream version of the drm_irq_install
function to be able to incorporate threaded irqs. So roll our own irq
install- and uninstall functions instead of relying on the drm core ones.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>