drm/fb_dma: Fix parameter name in htmldocs

drm/fb_dma: Fix parameter name in htmldocs

The parameter name is 'sb' and not 'drm_scanout_buffer'.
It fixes the following warnings:

drivers/gpu/drm/drm_fb_dma_helper.c:166: warning: Excess function parameter 'drm_scanout_buffer' description in 'drm_fb_dma_get_scanout_buffer'
drivers/gpu/drm/drm_fb_dma_helper.c:166: warning: Function parameter or struct member 'sb' not described in 'drm_fb_dma_get_scanout_buffer'
drivers/gpu/drm/drm_fb_dma_helper.c:166: warning: Excess function parameter 'drm_scanout_buffer' description in 'drm_fb_dma_get_scanout_buffer'

Fixes: 879b3b6511fe ("drm/fb_dma: Add generic get_scanout_buffer() for drm_panic")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240416090601.237286-1-jfalempe@redhat.com