drm: move native byte order quirk to new drm_driver_legacy_fb_format function

Turns out we need the pixel format fixup not only for the addfb ioctl,
but also for fbdev emulation code.

Ideally we would place it in drm_mode_legacy_fb_format().  That would
create alot of churn though, and most drivers don't care because they
never ever run on a big endian platform.  So add a new
drm_driver_legacy_fb_format() function instead which looks at the
mode_config->quirk_addfb_prefer_host_byte_order flag.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20180921134704.12826-2-kraxel@redhat.com