drm: rcar-du: Fix crash when no CMM is available

drm: rcar-du: Fix crash when no CMM is available

Commit 3bce3fdd1ff2 ("drm: rcar-du: Don't leak device_link to CMM")
refactored CMM handling, and introduced an incorrect test for CMM
availability. When no CMM is present, the rcrtc->cmm field is NULL,
testing rcrtc->cmm->dev causes a NULL pointer dereference. This slipped
through testing as all tests were run with the CMM present.

Fix this issue by correctly testing for rcrtc->cmm.

Fixes: 3bce3fdd1ff2 ("drm: rcar-du: Don't leak device_link to CMM")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Closes: https://lore.kernel.org/dri-devel/CAMuHMdXomz9GFDqkBjGX9Sda_GLccPcrihvFbOz0GAitDVNTbw@mail.gmail.com
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/20260408124205.1962448-1-laurent.pinchart+renesas@ideasonboard.com
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>