drm/format_helper: fix a kernel-doc typo

It looks like the incorrect name of a function parameter was used
in the kernel-doc notation, so just change it to the function's
parameter name to quell the kernel-doc warning.

drivers/gpu/drm/drm_format_helper.c:640: warning: Function parameter or member 'vaddr' not described in 'drm_fb_xrgb8888_to_mono_reversed'
drivers/gpu/drm/drm_format_helper.c:640: warning: Excess function parameter 'src' description in 'drm_fb_xrgb8888_to_mono_reversed'

Fixes: bcf8b616

 ("drm/format-helper: Add drm_fb_xrgb8888_to_mono_reversed()")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
CC: Maxime Ripard <mripard@kernel.org>
CC: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/480560/