drm: Build drm_irq.o only if CONFIG_DRM_LEGACY has been set

All code in drm_irq.o is for legacy UMs drivers. Only build and link
the file if CONFIG_DRM_LEGACY has been enabled.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020131941.15367-2-tzimmermann@suse.de