drm: Fix irq install error handling

The registered irq should be unregistered by free_irq() if
irq_postinstall() returns the error after request_irq() is called
successfully.

[airlied: add vga switcheroo disable]

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>