drm/udl: Use new drm_fb_helper functions

Use the newly created wrapper drm_fb_helper functions instead of calling
core fbdev functions directly. They also simplify the fb_info creation.

v2:
- remove unused variable device in udlfb_create

Cc: David Airlie <airlied@linux.ie>
Cc: Haixia Shi <hshi@chromium.org>
Cc: "Stéphane Marchesin" <marcheu@chromium.org>

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>