drm: Remove unused devm_drm_irq_install()

DRM IRQ helpers will become legacy. The function devm_drm_irq_install()
is unused and won't be required later.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210803090704.32152-14-tzimmermann@suse.de