drm: IRQ midlayer is now legacy

Hide the DRM midlayer behind CONFIG_DRM_LEGACY, make functions use
the prefix drm_legacy_, and move declarations to drm_legacy.h.
In struct drm_device, move the fields irq and irq_enabled behind
CONFIG_DRM_LEGACY.

All callers have been updated.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210803090704.32152-15-tzimmermann@suse.de