drm/fbdev-dma: Fix documented default preferred_bpp value

As of commit 6c80a93b ("drm/fb-helper: Initialize fb-helper's
preferred BPP in prepare function"), the preferred_bpp parameter of
drm_fb_helper_prepare() defaults to 32 instead of
drm_mode_config.preferred_depth.  Hence this also applies to
drm_fbdev_dma_setup(), which just passes its own preferred_bpp
parameter.

Fixes: b79fe9ab ("drm/fbdev-dma: Implement fbdev emulation for GEM DMA helpers")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/91f093ffe436a9f94d58fb2bfbc1407f1ebe8bb0.1688656591.git.geert+renesas@glider.be