Drop the DRM IRQ midlayer in favor of Linux IRQ interfaces. DRM's
IRQ helpers are mostly useful for UMS drivers. Modern KMS drivers
don't benefit from using it.

v3:
	* return error if (ret < 0) (Geert)
	* remove duplicate error message (Geert)
v2:
	* handle errors in platform_get_irq() (Geert, Sergei)
	* store IRQ number in struct shmob_drm_device (Laurent)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210720080941.23646-1-tzimmermann@suse.de