drm/vram-helper: Fix 'multi-line' kernel-doc comments

Reformat lines in kernel-doc comments, which make use of the backslash at
the end to suggest it is a multi-line comment. kernel-doc is able to
process e.g. the short description of a function properly, even if it is
across two lines.

No functional change.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240122093152.22536-2-anna-maria@linutronix.de