drm/cirrus: Use devm_drm_dev_alloc

Already using devm_drm_dev_init, so very simple replacment.

Acked-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: virtualization@lists.linux-foundation.org
Cc: Emil Velikov <emil.velikov@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-36-daniel.vetter@ffwll.ch