drm/vblank: Fix kernel docs for vblank timer

drm/vblank: Fix kernel docs for vblank timer

Fix documentation for drm_crtc_vblank_start_timer(), which referred
to drm_crtc_vblank_cancel_timer().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/dri-devel/20251106152201.6f248c09@canb.auug.org.au/
Fixes: 74afeb812850 ("drm/vblank: Add vblank timer")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Louis Chauvet <louis.chauvet@bootlin.com>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://patch.msgid.link/20251106073207.11192-1-tzimmermann@suse.de