drm/fb-helper: Fix typo on kerneldoc

Copypasta mistake.

Fixes: 742547b7 ("drm/fb_helper: Create wrappers for blit, copyarea and fillrect funcs")
Cc: Archit Taneja <architt@codeaurora.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180524090105.21196-1-daniel.vetter@ffwll.ch