drm/shmobile: Convert to Linux IRQ interfaces

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