drm: use drm_driver_legacy_fb_format in drm_gem_fbdev_fb_create

Creating framebuffers for fbdev emulation should use the correct format
code too, so switch drm_gem_fbdev_fb_create() over to use the new
drm_driver_legacy_fb_format() function.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20180921134704.12826-3-kraxel@redhat.com