drm/bochs: support changing byteorder at mode set time

Add bochs_hw_set_*_endian() helper functions, to set the framebuffer
byteorder at mode set time.  Support both DRM_FORMAT_XRGB8888 and
DRM_FORMAT_BGRX8888 framebuffer formats, no matter what the native
machine byte order is.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20180921134704.12826-5-kraxel@redhat.com