drm/omapdrm: Remove outdated comment

The comment refers to drm_irq_install() et al, which are not used by
omapdrm. The functions are part of the DRM IRQ midlayer and shouldn't
be used any longer. Remove the comment.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210706073125.7689-1-tzimmermann@suse.de